[PR target/116256] Fix latent regression in pattern to associate arithmetic to simpli...

[PR target/116256] Fix latent regression in pattern to associate arithmetic to simplify constants

This is something I spotted working on an outstanding issue with pr116256.
It's a latent regression.   I'm reasonably sure that with some effort I could
find a testcase that would represent a regression, probably just by adjusting
the constant index in "f" within in gcc.c-torture/execute/index-1.c

We have to define_insn_and_split patterns that potentially reassociate shifts
and adds to make a constant term cheaper to synthesize. The split code for
these two patterns assumes that if the left shifted constant is cheaper than
the right shifted constant then the left shifted constant can be loaded with a
trivial set.  That is not always the case -- and we'll ICE.

This patch simplifies the matching condition so that it only matches when the
constant can be loaded with a single instruction.

Tested in my tester on rv32 and rv64.  Will wait for precommit CI to render a
verdict.

Jeff

	PR target/116256
gcc/
	* config/riscv/riscv.md (reassocating constant addition): Adjust
	condition to avoid creating an unrecognizable insn.