[PR target/115921] Improve reassociation for rv64

As Jovan pointed out in pr115921, we're not reassociating expressions like this
on rv64:

(x & 0x3e) << 12

It generates something like this:

        li      a5,258048
        slli    a0,a0,12
        and     a0,a0,a5

We have a pattern that's designed to clean this up.  Essentially reassociating
the operations so that we don't need to load the constant resulting in
something like this:

        andi    a0,a0,63
        slli    a0,a0,12

That pattern wasn't working for certain constants due to its condition. The
condition is trying to avoid cases where this kind of reassociation would
hinder shadd generation on rv64.  That condition was just written poorly.

This patch tightens up that condition in a few ways.  First, there's no need to
worry about shadd cases if ZBA is not enabled.  Second we can't use shadd if
the shift value isn't 1, 2 or 3.  Finally rather than open-coding one of the
tests, we can use an existing operand predicate.

The net is we'll start performing this transformation in more cases on rv64
while still avoiding reassociation if it would spoil shadd generation.

	PR target/115921
gcc/
	* config/riscv/riscv.md (reassociate bitwise ops): Tighten test for
	cases we do not want reassociate.

gcc/testsuite/
	* gcc.target/riscv/pr115921.c: New test.