[InstCombine] look through bitcasts to find selects

There was concern that creating bitcasts for the simpler potential select pattern:

define <2 x i64> @vecBitcastOp1(<4 x i1> %cmp, <2 x i64> %a) {
  %a2 = add <2 x i64> %a, %a
  %sext = sext <4 x i1> %cmp to <4 x i32>
  %bc = bitcast <4 x i32> %sext to <2 x i64>
  %and = and <2 x i64> %a2, %bc
  ret <2 x i64> %and
}

might lead to worse code for some targets, so this patch is matching the larger
patterns seen in the test cases.

The motivating example for this patch is this IR produced via SSE intrinsics in C:

define <2 x i64> @gibson(<2 x i64> %a, <2 x i64> %b) {
  %t0 = bitcast <2 x i64> %a to <4 x i32>
  %t1 = bitcast <2 x i64> %b to <4 x i32>
  %cmp = icmp sgt <4 x i32> %t0, %t1
  %sext = sext <4 x i1> %cmp to <4 x i32>
  %t2 = bitcast <4 x i32> %sext to <2 x i64>
  %and = and <2 x i64> %t2, %a
  %neg = xor <4 x i32> %sext, <i32 -1, i32 -1, i32 -1, i32 -1>
  %neg2 = bitcast <4 x i32> %neg to <2 x i64>
  %and2 = and <2 x i64> %neg2, %b
  %or = or <2 x i64> %and, %and2
  ret <2 x i64> %or
}

For an AVX target, this is currently:

vpcmpgtd  %xmm1, %xmm0, %xmm2
vpand     %xmm0, %xmm2, %xmm0
vpandn    %xmm1, %xmm2, %xmm1
vpor      %xmm1, %xmm0, %xmm0
retq

With this patch, it becomes:

vpmaxsd   %xmm1, %xmm0, %xmm0

Differential Revision: http://reviews.llvm.org/D20774

llvm-svn: 271676