[InstCombine] allow vector types for constant folding / computeKnownBits (PR24942)

computeKnownBits() already works for integer vectors, so allow vector types when calling that from InstCombine.

I don't think the change to use m_APInt in computeKnownBits is strictly necessary because we do check for 
ConstantVector later, but it's more efficient to handle the splat case without needing to loop on vector elements.

This should work with InstSimplify, but doesn't yet, so I made that a FIXME comment on the test for PR24942:
https://llvm.org/bugs/show_bug.cgi?id=24942

Differential Revision: https://reviews.llvm.org/D24677

llvm-svn: 281777