[X86] Improve mul w/ overflow codegen, to MUL8+SETO.

Currently, @llvm.smul.with.overflow.i8 expands to 9 instructions, where
3 are really needed.

This adds X86ISD::UMUL8/SMUL8 SD nodes, and custom lowers them to
MUL8/IMUL8 + SETO.

i8 is a special case because there is no two/three operand variants of
(I)MUL8, so the first operand and return value need to go in AL/AX.

Also, we can't write patterns for these instructions: TableGen refuses
patterns where output operands don't match SDNode results. In this case,
instructions where the output operand is an implicitly defined register.

A related special case (and FIXME) exists for MUL8 (X86InstrArith.td):

  // FIXME: Used for 8-bit mul, ignore result upper 8 bits.
  // This probably ought to be moved to a def : Pat<> if the
  // syntax can be accepted.
  [(set AL, (mul AL, GR8:$src)), (implicit EFLAGS)]

Ideally, these go away with UMUL8, but we still need to improve TableGen
support of implicit operands in patterns.

Before this change:
  movsbl  %sil, %eax
  movsbl  %dil, %ecx
  imull   %eax, %ecx
  movb    %cl, %al
  sarb    $7, %al
  movzbl  %al, %eax
  movzbl  %ch, %esi
  cmpl    %eax, %esi
  setne   %al

After:
  movb    %dil, %al
  imulb   %sil
  seto    %al

Also, remove a made-redundant testcase for PR19858, and enable more FastISel
ALU-overflow tests for SelectionDAG too.

Differential Revision: http://reviews.llvm.org/D5809