LoongArch: Relax ins_zero_bitmask_operand and remove and<mode>3_align

In r15-1207 I was too stupid to realize we just need to relax
ins_zero_bitmask_operand to allow using bstrins for aligning, instead of
adding a new split.  And, "> 12" in ins_zero_bitmask_operand also makes
no sense: it rejects bstrins for things like "x & ~4l" with no good
reason.

So fix my errors now.

gcc/ChangeLog:

	* config/loongarch/predicates.md (ins_zero_bitmask_operand):
	Cover more cases that bstrins can benefit.
	(high_bitmask_operand): Remove.
	* config/loongarch/constraints.md (Yy): Remove.
	* config/loongarch/loongarch.md (and<mode>3_align): Remove.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/bstrins-4.c: New test.