[InstCombine] Remove 64-bit bit width restriction from m_ConstantInt(uint64_t*&)

I think we only need to make sure the value fits in 64-bits not that bit width is 64-bit.

This helps places that use this for shift amounts since the shift amount needs to be the same bitwidth as the LHS, but can't be larger than the bit width.

Differential Revision: https://reviews.llvm.org/D34737

llvm-svn: 306577