cmd/compile: fold constants into immediate instructions on riscv64

Where possible, fold constants into versions of instructions that take
an immediate. This avoids the need to allocate a register and load the
immediate into it.

Change-Id: If911ca41235e218490679aed2ce5f48bf807a2b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/222639


Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>