Fix SGPR + VGPR + offset Scratch offset folding

Values in SGPR and VGPR register are treated as unsigned by hardware.

When value in 32-bit SGPR or VGPR base can be negative calculate offset
using 32-bit add instructions, otherwise use
sgpr(unsigned) + vgpr(unsigned) + offset.

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

Differential Revision: https://reviews.llvm.org/D144957