[RISCV] Fix stackmap shadow trimming NOP size for compressed targets (#189774)
The shadow trimming loop in LowerSTACKMAP hardcoded a 4-byte decrement
per instruction, but when Zca is enabled NOPs are 2 bytes. Use NOPBytes
instead of the hardcoded 4 so the shadow is correctly trimmed on
compressed targets.
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>