R600/SI: Assign a register class to the $vaddr operand for MIMG instructions

The previous code declared the operand as unknown:$vaddr, which made
it possible for scalar registers to be used instead of vector registers.
R600/SI: Choose the correct MOV instruction for copying immediates

The instruction selector will now try to infer the destination register
so it can decided whether to use V_MOV_B32 or S_MOV_B32 when copying
immediates.
R600/SI: Lower BUILD_VECTOR to REG_SEQUENCE v2

Using REG_SEQUENCE for BUILD_VECTOR rather than a series of INSERT_SUBREG
instructions should make it easier for the register allocator to coalasce
unnecessary copies.

v2:
  - Use an SGPR register class if all the operands of BUILD_VECTOR are
    SGPRs.
R600/SI: Use i8 types for resource descriptors in tests

We switched from i32 to i8 types a while ago and the tests were never
updated.
R600/SI: Convert v16i8 resource descriptors to i128

Now that compute support is better on SI, we can't continue using v16i8
for descriptors since this is also a legal type in OpenCL.

This patch fixes numerous hangs with the piglit OpenCL test and since
we now use a target specific DAG node for LOAD_CONSTANT with the
correct MemOperandFlags, this should also fix:

https://bugs.freedesktop.org/show_bug.cgi?id=66805
R600/SI: Replace v1i32 type with i32 in imageload and sample intrinsics