LoongArch: Fix wrong code with <optab>_alsl_reversesi_extended

LoongArch: Fix wrong code with <optab>_alsl_reversesi_extended

The second source register of this insn cannot be the same as the
destination register.

gcc/ChangeLog:

	* config/loongarch/loongarch.md
	(<optab>_alsl_reversesi_extended): Add '&' to the destination
	register constraint and append '0' to the first source register
	constraint to indicate the destination register cannot be same
	as the second source register, and change the split condition to
	reload_completed so that the insn will be split only after RA in
	order to obtain allocated registers that satisfy the above
	constraints.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/bitwise-shift-reassoc-clobber.c: New
	test.