[AMDGPU] Consider all SGPR uses as unique in constant bus verify

Fix the verifier so that overlapping SGPR operands are counted
independently.  We cannot assume that overlapping SGPR accesses
only count as a single constant bus use.
The exception is implicit uses which do not add to constant bus
usage (only) when overlapping.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D87748