RISC-V: Expand const_vector with 2 elts per pattern.

RISC-V: Expand const_vector with 2 elts per pattern.

Hi,

In PR121334 we are asked to expand a const_vector of size 4 with
poly_int elements.  It has 2 elts per pattern so is neither a
const_vector_duplicate nor a const_vector_stepped.

We don't allow this kind of constant in legitimate_constant_p but expr
apparently still wants us to expand it under certain conditions.

This patch implements a basic expander for such kinds of patterns.
As slide1up is used to build the individual vectors it also adds
a helper function expand_slide1up.

I regtested on rv64gcv_zvl512b but unfortunately the newly created pattern is
not even executed.  I tried some variations of the original code but didn't
manage to trigger it.

Regards
 Robin

	PR target/121334

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (expand_slide1up): New function.
	(expand_vector_init_trailing_same_elem): Use new function.
	(expand_const_vector_onestep): New function.
	(expand_const_vector): Uew expand_slide1up.
	(expand_vector_init_merge_repeating_sequence): Ditto.
	(shuffle_off_by_one_patterns): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr121334.c: New test.