LoongArch: Optimize statement to use bstrins.{w|d}

LoongArch: Optimize statement to use bstrins.{w|d}

For statement (a << imm1) | (b & imm2), in case the imm2 equals to
(1 << imm1) - 1, it can be optimized to use bstrins.{w|d} instruction.

gcc/ChangeLog:

	* config/loongarch/loongarch.md
	(*bstrins_w_for_ior_ashift_and_extend): New template.
	(*bstrins_d_for_ior_ashift_and): New template.
	* config/loongarch/predicates.md (const_uimm63_operand): New
	predicate.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/bstrins-5.c: New test.
	* gcc.target/loongarch/bstrins-6.c: New test.