[InstCombine] Teach foldSelectICmpAnd to recognize a (icmp slt X, 0) and (icmp...

[InstCombine] Teach foldSelectICmpAnd to recognize a (icmp slt X, 0) and (icmp sgt X, -1) as equivalent to an and with the sign bit of the truncated type

This is similar to what was already done in foldSelectICmpAndOr. Ultimately I'd like to see if we can call foldSelectICmpAnd from foldSelectIntoOp if we detect a power of 2 constant. This would allow us to remove foldSelectICmpAndOr entirely.

Differential Revision: https://reviews.llvm.org/D36498

llvm-svn: 311362