cmd/compile: adjust RISCV64 rewrite rules to use typed aux fields

Also add a typed version of mergeSym to rewrite.go to assist with a few
rules that used mergeSym in the untyped-form.

Remove a few extra int32 overflow checks that no longer make sense, as
adding two int8s or int16s should never overflow an int32.

Passes toolstash-check -all.

Change-Id: I72ddd2b0d9001faa87ad0ab54f500057164661b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/228882
Reviewed-by: Keith Randall <[email protected]>

Author: dfinkel