TEST: Fix dump FAIL for RVV (RISCV-V vector)

TEST: Fix dump FAIL for RVV (RISCV-V vector)

As this showed: https://godbolt.org/z/3K9oK7fx3

ARM SVE 2 times for FOLD_EXTRACT_LAST wheras RVV 4 times.

This is because RISC-V doesn't enable vec_pack_trunc so we will failed conversion and fold_extract_last at the first time analysis.
Then we succeed at the second time.

So RVV has 4 times of showing "FOLD_EXTRACT_LAST:.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/vect-cond-reduc-4.c: Add vect_pack_trunc variant.