arm: fix rtl checking bootstrap (PR target/93312)

arm: fix rtl checking bootstrap (PR target/93312)

As reported in PR93312, the:
> > > > > >         * config/arm/arm.c (clear_operation_p): New function.
change broke RTL checking bootstrap.

On the testcase from the PR (which is distilled from libgcc2.c, so I think
we don't need to add it into testsuite) we ICE because SET_DEST (elt) is
not a REG, but SUBREG.  The code uses REGNO on it, which is invalid, but
only stores it into a variable, then performs REG_P (reg) check,
determines it is not a REG and bails early.

The following patch just moves the regno variable initialization after that
check, it isn't used in between.  And, as a small optimization, because
reg doesn't change, doesn't use REGNO (reg) a second time to set last_regno.

2020-01-18  Jakub Jelinek  <jakub@redhat.com>

	PR target/93312
	* config/arm/arm.c (clear_operation_p): Don't use REGNO until
	after checking the argument is a REG.  Don't use REGNO (reg)
	again to set last_regno, reuse regno variable instead.