cmd/compile: improve Eq32/Neq32 on riscv64

cmd/compile: improve Eq32/Neq32 on riscv64

Use SUBW to perform a 32-bit subtraction, rather than zero extending from
32 to 64 bits. This reduces Eq32 and Neq32 to two instructions, rather than
the four instructions required previously.

Change-Id: Ib2798324881e9db842c864e91a0c1b1e48c4b67b
Reviewed-on: https://go-review.googlesource.com/c/go/+/220921
Reviewed-by: Cherry Zhang <cherryyz@google.com>