[x86] Improve select lowering for smin(x, 0) & smax(x, 0)

smin(x, 0):
  (select (x < 0), x, 0) -> ((x >> (size_in_bits(x)-1))) & x

smax(x, 0):
  (select (x > 0), x, 0) -> (~(x >> (size_in_bits(x)-1))) & x
  The comparison is testing for a positive value, we have to invert the sign
  bit mask, so only do that transform if the target has a bitwise 'and not'
  instruction (the invert is free).

The transform is performed only when CMP has a single user to avoid
increasing total instruction number.

https://alive2.llvm.org/ce/z/euUnNm
https://alive2.llvm.org/ce/z/37339J

Differential Revision: https://reviews.llvm.org/D123109