RISC-V: Disable two-source permutes for now [PR117173].

RISC-V: Disable two-source permutes for now [PR117173].

After testing on the BPI (4.2% improvement for x264 input 1, 4.4% for
input 2) and the discussion in PR117173 I figured it's best to disable
the two-source permutes by default for now.

The patch adds a parameter "riscv-two-source-permutes" which restores
the old behavior.

	PR target/117173

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (shuffle_generic_patterns): Only
	support single-source permutes by default.
	* config/riscv/riscv.opt: New param "riscv-two-source-permutes".

gcc/testsuite/ChangeLog:

	* gcc.dg/fold-perm-2.c: Run with two-source permutes.
	* gcc.dg/pr54346.c: Ditto.