RISC-V: Fix vec_init for simple sequences [PR114028].

For a vec_init (_a, _a, _a, _a) with _a of mode DImode we try to
construct a "superword" of two "_a"s.  This only works for modes < Pmode
when we can "shift and or" both halves into one Pmode register.
This patch disallows the optimization for inner_mode == Pmode and emits
a simple broadcast in such a case.

gcc/ChangeLog:

	PR target/114028

	* config/riscv/riscv-v.cc (rvv_builder::can_duplicate_repeating_sequence_p):
	Return false if inner mode is already Pmode.
	(rvv_builder::is_all_same_sequence): New function.
	(expand_vec_init): Emit broadcast if sequence is all same.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr114028.c: New test.