Restore building for SH after bswap related changes

Recent changes result in triggering a compile-time error if the bswap pattern
FAILs.  This tripped on the SH port where one of the bswap patterns FAILs if we
can't create new pseudos.  We can restore building for SH by moving the check
into the pattern's condition.

Tested by building sh3-linux-gnu/sh3eb-linux-gnu where the compiler builds
again and there are no testsuite regressions relative to the last clean build a
few days ago.  sh4/sh4eb, which bootstrap, will run sometime over the coming
days.

Pushing to the trunk.

gcc/
	* config/sh/sh.md (bswapsi2): Move check for can_create_pseudos_p
	into condition rather than FAILing.