[CGP] avoid zext/trunc of a memcmp expansion compare

This could be viewed as another shortcoming of the DAGCombiner:
when both operands of a compare are zexted from the same source
type, we should be able to compare the original types.

The effect on PowerPC perf is likely unnoticeable, but there's a
visible regression for x86 if we feed the suboptimal IR for memcmp
expansion to the DAG:

_cmp_eq4_zexted_to_i64:
  movl  (%rdi), %ecx
  movl  (%rsi), %edx
  xorl  %eax, %eax
  cmpq  %rdx, %rcx
  sete  %al

_cmp_eq4_better:
  movl  (%rdi), %ecx
  xorl  %eax, %eax
  cmpl  (%rsi), %ecx
  sete  %al

llvm-svn: 304923