[InstSimplify] fold nested min/max intrinsics with constant operands

This is based on the existing code for the non-intrinsic idioms
in InstCombine.

The vector constant constraint is non-obvious: undefs should be
ok in the outer call, but they can't propagate safely from the
inner call in all cases. Example:

https://alive2.llvm.org/ce/z/-2bVbM
  define <2 x i8> @src(<2 x i8> %x) {
  %0:
    %m = umin <2 x i8> %x, { 7, undef }
    %m2 = umin <2 x i8> { 9, 9 }, %m
    ret <2 x i8> %m2
  }
  =>
  define <2 x i8> @tgt(<2 x i8> %x) {
  %0:
    %m = umin <2 x i8> %x, { 7, undef }
    ret <2 x i8> %m
  }
  Transformation doesn't verify!
  ERROR: Value mismatch

  Example:
  <2 x i8> %x = < undef, undef >

  Source:
  <2 x i8> %m = < #x00 (0)	[based on undef value], #x00 (0) >
  <2 x i8> %m2 = < #x00 (0), #x00 (0) >

  Target:
  <2 x i8> %m = < #x07 (7), #x10 (16) >
  Source value: < #x00 (0), #x00 (0) >
  Target value: < #x07 (7), #x10 (16) >