[ValueTracking] recognize obfuscated variants of umin/umax

The smallest tests that expose this are codegen tests (because SelectionDAGBuilder::visitSelect() uses matchSelectPattern
to create UMAX/UMIN nodes), but it's also possible to see the effects in IR alone with folds of min/max pairs.

If these were written as unsigned compares in IR, InstCombine canonicalizes the unsigned compares to signed compares. 
Ie, running the optimizer pessimizes the codegen for this case without this patch:

define <4 x i32> @umax_vec(<4 x i32> %x) {
  %cmp = icmp ugt <4 x i32> %x, <i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647>
  %sel = select <4 x i1> %cmp, <4 x i32> %x, <4 x i32> <i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647>
  ret <4 x i32> %sel
}

$ ./opt umax.ll -S | ./llc -o - -mattr=avx

vpmaxud LCPI0_0(%rip), %xmm0, %xmm0

$ ./opt -instcombine umax.ll -S | ./llc -o - -mattr=avx

vpxor %xmm1, %xmm1, %xmm1
vpcmpgtd  %xmm0, %xmm1, %xmm1
vmovaps LCPI0_0(%rip), %xmm2    ## xmm2 = [2147483647,2147483647,2147483647,2147483647]
vblendvps %xmm1, %xmm0, %xmm2, %xmm0

Differential Revision: https://reviews.llvm.org/D26096

llvm-svn: 286318