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.