cmd/internal/obj/mips: fix encoding of FCR registers

The asm encoder generally assumes that the lowest 5 bits of the
REG_XX constants match the machine instruction encoding, i.e.
the lowest 5 bits is the register number. This was not true for
FCR registers and M registers. Make it so.

MOV Rx, FCRy was encoded as two machine instructions. The first
is unnecessary. Remove.

Change-Id: Ib988e6b109ba8f564337cdd31019c1a6f1881f5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/203717


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>