[RISCV] GISel custom lowering for G_ADD/G_SUB (#121587)
Custom lowering for s32 G_ADD/SUB to help match selection dag better.
Specifically for RV64 a s32 is produced as a add+sext the output this
allows for fewer instructions to sign extend a couple patterns. Allows
for the generation of addiw,subw,negw to reduce required instructions to
load values quicker
Log2_ceil_i32 in rvzbb.ll shows a more obvious improvement case.
