Fix SGPR + offset Scratch offset folding

Values in SGPR register are treated as unsigned by hardware.

When value in 32-bit SGPR base can be negative calculate offset using
32-bit add instruction, otherwise use sgpr base(unsigned) + offset.
Does not affect case where whole offset comes from SGPR register
(immediate offset is 0).

LoopStrengthReduce.cpp changes offsets to negative and in some
iterations value in SGPR register could be negative.

Differential Revision: https://reviews.llvm.org/D144955