[DAGCombiner] fold select-of-constants based on sign-bit test

Examples:
  i32 X > -1 ? C1 : -1 --> (X >>s 31) | C1
  i8 X < 0 ? C1 : 0 --> (X >>s 7) & C1

This is a small generalization of a fold requested in PR43650:
https://bugs.llvm.org/show_bug.cgi?id=43650

The sign-bit of the condition operand can be used as a mask for the true operand:
https://rise4fun.com/Alive/paT

Note that we already handle some of the patterns (isNegative + scalar) because
there's an over-specialized, yet over-reaching fold for that in foldSelectCCToShiftAnd().
It doesn't use any TLI hooks, so I can't easily rip out that code even though we're
duplicating part of it here. This fold is guarded by TLI.convertSelectOfConstantsToMath(),
so it should not cause problems for targets that prefer select over shift.

Also worth noting: I thought we could generalize this further to include the case where
the true operand of the select is not constant, but Alive says that may allow poison to
pass through where it does not in the original select form of the code.

Differential Revision: https://reviews.llvm.org/D68949

llvm-svn: 374902