cmd/internal/obj/riscv: clean up immediate checking
Change immIFits to return an error in the case that it does not fit.
This allows for deduplication and consistency of error messages.
Additionally, since we've already calculated the min and max values,
we can easily include these in the message. Also provide and use
immEven, for the same reasons.
Change-Id: Ie680558744f3e9bc19d6913c4144ce9ddbd0429c
Reviewed-on: https://go-review.googlesource.com/c/go/+/523458 Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Run-TryBot: M Zhuo <mzh@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: M Zhuo <mzh@golangcn.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com>