LoongArch: Split vec_selects of bottom elements into simple move

LoongArch: Split vec_selects of bottom elements into simple move

For below pattern, can be treated as a simple move because floating point
and vector share a common register on loongarch64.

(set (reg/v:SF 32 $f0 [orig:93 res ] [93])
      (vec_select:SF (reg:V8SF 32 $f0 [115])
          (parallel [
                  (const_int 0 [0])
              ])))

gcc/ChangeLog:

	* config/loongarch/lasx.md (vec_extract<mode>_0):
	New define_insn_and_split patten.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/vect-extract.c: New test.