[committed] [RISC-V] Trivial pattern cleanup

[committed] [RISC-V] Trivial pattern cleanup

As I was reviewing and cleaning up some internal work, I noticed a particular
idiom being used elsewhere in the RISC-V backend.

Specifically the use of explicit subregs when an adjustment to the
match_operand would be sufficient.

Let's take this example from the and-not splitter:

>  (define_split
>    [(set (match_operand:X 0 "register_operand")
>         (and:X (not:X (lshiftrt:X (match_operand:X 1 "register_operand")
>                                   (subreg:QI (match_operand:X 2 "register_operand") 0)))
>                (const_int 1)))]

Note the explicit subreg.  We can instead use a match_operand with QImode.
This ever-so-slightly simplifies the machine description.

It also means that if we have a QImode object lying around (say we loaded it
from memory in QImode), we can use it directly rather than first extending it
to X, then truncing to QI.  So we end up with simpler RTL and in rare cases
improve the code we generate.

When used in a define_split or define_insn_and_split we need to make suitable
adjustments to the split RTL.

Bootstrapped a while back.  Just re-tested with a cross.

gcc/
	* config/riscv/bitmanip.md (splitter to use w-form division): Remove
	explicit subregs.
	(zero extended bitfield extraction): Similarly.
	* config/riscv/thead.md (*th_memidx_operand): Similarly.