cmd/internal/obj/ppc64: fix mtocrf, cleanup other CR ops

Fix "MOVW CRx, Rx" and "MOVFL Rx, constant", The FXM field was not
encoded correctly.

Generate mtocrf instead of mtcrf when a CRx argument is used. This
form is much faster.

Simplify several conditional statements which test if the register
argument is REG_CR or one of REG_CRx if the tested argument is known
to be matched as C_CREG. Likewise, a4 is (the From3 arg) is always
TYPE_NONE in the existing optab entries for type_ 69.

Change-Id: I3a4749b1cbfdfab6a2616586ae59e932e01dae50
Reviewed-on: https://go-review.googlesource.com/c/go/+/352789


Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>