x64: lower isub by a constant to lea

[darmie]

iadd of a value and a 32/64-bit constant already lowers to lea, folding
the constant into the address displacement and computing the result in a
different register than the input without a separate mov. isub by a
constant did not have a matching rule: it fell through to the two-operand
sub, which forces a mov+sub pair whenever the destination register
differs from the source.

This adds a rule that lowers x - C (for a 32- or 64-bit integer constant
C) to lea -C(x), mirroring the existing iadd path. The rule fires only
when the negated constant fits in an Offset32 (i.e. C is not i32::MIN);
register/register isub is unchanged.

Example

function %isub_c2(i64) -> i64 {
block0(v0: i64):
    v1 = iconst.i64 2
    v2 = isub v0, v1
    return v2
}

Before:

movq %rdi, %rax
subq $2, %rax

After:

leaq -2(%rdi), %rax

One instruction instead of two, and no flags clobbered.

Testing

• New precise-output filetest (isub-const-lea.clif) asserting the lea
  lowering for i64 and i32, and that register/register isub still lowers
  to sub.
• New runtest exercising x - C (including a negative constant) on the
  interpreter and every native target.
• The existing isa/x64 filetests pass unchanged.

I found this while profiling a call/arithmetic-heavy workload where every
argument setup was an x - C; folding the mov away was a measurable win.

[darmie]

Pushed a fixup: refreshed the 12 load-store/x64 disas goldens that the CLI disas test flagged. Those are dynamic-memory bounds checks that compute bound - access_size in place; with this rule that sub $C now goes through lea and degenerates to add $-C — the same instruction count, and identical to how iadd by a constant already lowers today (verified isub v, C and iadd v, -C produce byte-identical output). The win over the old mov+sub shows up when the result lands in a different register than the input.

[github-actions]

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[cfallin]

Hi @darmie -- are you aware of the add-vs-lea performance discussion thread we had recently (#13325)? I ask because at the very least, we should benchmark this change with the full Sightglass suite. I am also pretty leery in general of introducing more uses of LEA given the performance variability that we've seen on different systems. I'm not surprised that you saw a speedup on one particular benchmark since LEA is "non-destructive" (doesn't clobber the source) but I'd be curious to know how this looks overall. Thanks!

[darmie]

Hi @cfallin! Actually I am not aware of that thread, I should have searched first. Yes I saw a performance improvement on my alderlake x86_64 box, but it's only ~6.9% so far. I'll read through the thread now.