riscv: `master` saves callee-saved registers unexpectedly on ReleaseFast mode

[omerfirmak]

Zig Version

all versions after 0.13.0

Steps to Reproduce and Observed Behavior

Can simply be observed on https://godbolt.org/z/PW7faeY6M

When in ReleaseFast compilation mode, zig thinks that ra and s0 should be saved for no apparent reason. ReleaseSmall is able to generate the expected assembly with just a mul and a ret. This appears to be a regression introduced in 0.13.0, since 0.13.0 is able to generate the expected assembly with both ReleaseFast and ReleaseSmall.

Expected Behavior

ReleaseFast mode should match the assembly in ReleaseSmall mode.

[alexrp]

What you're seeing is frame pointer maintenance. Use -fomit-frame-pointer if you don't want this; see #22180.