[InstCombine] fold select X, (ext X), C

If we're going to canonicalize IR towards select of constants, try harder to create those.
Also, don't lose the metadata.

This is actually 4 related transforms in one patch:
      // select X, (sext X), C --> select X, -1, C
      // select X, (zext X), C --> select X,  1, C
      // select X, C, (sext X) --> select X, C, 0
      // select X, C, (zext X) --> select X, C, 0

Differential Revision: https://reviews.llvm.org/D25126

llvm-svn: 283575