[AMDGPU] Change numBitsSigned for simplicity and document it. NFC.

Change numBitsSigned to return the minimum size of a signed integer that
can hold the value. This is different by one from the previous result
but is more consistent with numBitsUnsigned. Update all callers. All
callers are now more consistent between the signed and unsigned cases,
and some callers get simpler, especially the ones that deal with
quantities like numBitsSigned(LHS) + numBitsSigned(RHS).

Differential Revision: https://reviews.llvm.org/D112813