RISC-V: Enable Hoist to GCSE simple constants

Hoist want_to_gcse_p () calls rtx_cost () to compute max distance for
hoist candidates. For a simple const (say 6 which needs seperate insn "LI 6")
backend currently returns 0, causing Hoist to bail and elide GCSE.

Note that constants requiring more than 1 insns to setup were working
fine since riscv_rtx_costs () was returning non-zero (although that
itself might need refining: see bugzilla 111139).

To keep testsuite parity, some V tests need updating which started failing
in the new costing regime.

gcc/ChangeLog:
	* config/riscv/riscv.cc (riscv_rtx_costs): Adjust const_int
	cost. Add some comments about different constants handling.

gcc/testsuite/ChangeLog:
	* gcc.target/riscv/gcse-const.c: New Test
	* gcc.target/riscv/rvv/vsetvl/vlmax_conflict-7.c: Remove test
	for Jump.
	* gcc.target/riscv/rvv/vsetvl/vlmax_conflict-8.c: Ditto.

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>

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