LoongArch: Provide ashr lshr and ashl RTL pattern for vectors.

LoongArch: Provide ashr lshr and ashl RTL pattern for vectors.

We support vashr vlshr and vashl. However, in r15-1638 support optimize
x < 0 ? -1 : 0 into (signed) x >> 31 and x < 0 ? 1 : 0 into (unsigned) x >> 31.
To support this optimization, vector ashr lshr and ashl need to be implemented.

gcc/ChangeLog:

	* config/loongarch/loongarch.md (insn): Added rotatert rotr pairs.
	* config/loongarch/simd.md (rotr<mode>3): Remove to ...
	(<optab><mode>3): This.

gcc/testsuite/ChangeLog:

	* g++.target/loongarch/vect-ashr-lshr.C: New test.