LoongArch: Handle vectorized copysign (x, -1) expansion efficiently

LoongArch: Handle vectorized copysign (x, -1) expansion efficiently

With LSX or LASX, copysign (x[i], -1) (or any negative constant) can be
vectorized using [x]vbitseti.{w/d} instructions to directly set the
signbits.

Inspired by Tamar Christina's "AArch64: Handle copysign (x, -1) expansion
efficiently" (r14-5289).

gcc/ChangeLog:

	* config/loongarch/lsx.md (copysign<mode>3): Allow operand[2] to
	be an reg_or_vector_same_val_operand.  If it's a const vector
	with same negative elements, expand the copysign with a bitset
	instruction.  Otherwise, force it into an register.
	* config/loongarch/lasx.md (copysign<mode>3): Likewise.

gcc/testsuite/ChangeLog:

	* g++.target/loongarch/vect-copysign-negconst.C: New test.
	* g++.target/loongarch/vect-copysign-negconst-run.C: New test.