AArch64: Constant fold converting vector setcc results to float.

Since the result of a SETCC for AArch64 is 0 or -1 in each lane, we can
move unary operations, in this case [su]int_to_fp through the mask
operation and constant fold the operation away. Generally speaking:
  UNARYOP(AND(VECTOR_CMP(x,y), constant))
      --> AND(VECTOR_CMP(x,y), constant2)
where constant2 is UNARYOP(constant).

This implements the transform where UNARYOP is [su]int_to_fp.

For example, consider the simple function:
define <4 x float> @foo(<4 x float> %val, <4 x float> %test) nounwind {
  %cmp = fcmp oeq <4 x float> %val, %test
  %ext = zext <4 x i1> %cmp to <4 x i32>
  %result = sitofp <4 x i32> %ext to <4 x float>
  ret <4 x float> %result
}

Before this change, the code is generated as:
  fcmeq.4s  v0, v0, v1
  movi.4s v1, #0x1        // Integer splat value.
  and.16b v0, v0, v1      // Mask lanes based on the comparison.
  scvtf.4s  v0, v0        // Convert each lane to f32.
  ret

After, the code is improved to:
  fcmeq.4s  v0, v0, v1
  fmov.4s v1, #1.00000000 // f32 splat value.
  and.16b v0, v0, v1      // Mask lanes based on the comparison.
  ret

The svvtf.4s has been constant folded away and the floating point 1.0f
vector lanes are materialized directly via fmov.4s.

Rather than do the folding manually in the target code, teach getNode()
in the generic SelectionDAG to handle folding constant operands of
vector [su]int_to_fp nodes. It is reasonable (as noted in a FIXME) to do
additional constant folding there as well, but I don't have test cases
for those operations, so leaving them for another time when it becomes
appropriate.

rdar://17693791
X86: Constant fold converting vector setcc results to float.

Since the result of a SETCC for X86 is 0 or -1 in each lane, we can
move unary operations, in this case [su]int_to_fp through the mask
operation and constant fold the operation away. Generally speaking:
  UNARYOP(AND(VECTOR_CMP(x,y), constant))
      --> AND(VECTOR_CMP(x,y), constant2)
where constant2 is UNARYOP(constant).

This implements the transform where UNARYOP is [su]int_to_fp.

For example, consider the simple function:
define <4 x float> @foo(<4 x float> %val, <4 x float> %test) nounwind {
  %cmp = fcmp oeq <4 x float> %val, %test
  %ext = zext <4 x i1> %cmp to <4 x i32>
  %result = sitofp <4 x i32> %ext to <4 x float>
  ret <4 x float> %result
}

Before this change, the SSE code is generated as:
LCPI0_0:
  .long 1                       ## 0x1
  .long 1                       ## 0x1
  .long 1                       ## 0x1
  .long 1                       ## 0x1
  .section  __TEXT,__text,regular,pure_instructions
  .globl  _foo
  .align  4, 0x90
_foo:                                   ## @foo
  cmpeqps %xmm1, %xmm0
  andps LCPI0_0(%rip), %xmm0
  cvtdq2ps  %xmm0, %xmm0
  retq

After, the code is improved to:
LCPI0_0:
  .long 1065353216              ## float 1.000000e+00
  .long 1065353216              ## float 1.000000e+00
  .long 1065353216              ## float 1.000000e+00
  .long 1065353216              ## float 1.000000e+00
  .section  __TEXT,__text,regular,pure_instructions
  .globl  _foo
  .align  4, 0x90
_foo:                                   ## @foo
  cmpeqps %xmm1, %xmm0
  andps LCPI0_0(%rip), %xmm0
  retq

The cvtdq2ps has been constant folded away and the floating point 1.0f
vector lanes are materialized directly via the ModRM operand of andps.