aarch64: Use VCodeConstant for f64/v128 constants
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I was looking into `constant_f128` and its fallback lowering into memory
and to get familiar with the code I figured it'd be good to port some
Rust logic to ISLE. This commit ports the `constant_{f128,f64,f32}`
helpers into ISLE from Rust as well as the `splat_const` helper which
ended up being closely related.
Tests reflect a number of regalloc changes that happened but also namely
one major difference is that in the lowering of `f32` a 32-bit immediate
is created now instead of a 64-bit immediate (in a GP register before
it's moved into a FP register). This semantically has no change but the
generated code is slightly different in a few minor cases.
This commit removes the `LoadFpuConst64` and `LoadFpuConst128` pseudo-instructions from the AArch64 backend which internally loaded a nearby constant and then jumped over it. Constants now go through the `VCodeConstant` infrastructure which gets placed at the end of the function similar to how x64 works. Some minor support was added in as well to add a new addressing mode for a `MachLabel`-relative load.
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As was pointed out recently one minor improvement to the AArch64 backend is to use the same infrastructure that x64 uses for loading constants which is to pool them at the end of the function instead of having them inline in the body. This commit implements this strategy to ensure that fallback
shuffleinstructions, for example, don't generate a branch instruction to skip over immediates.Additionally in these commits is a refactoring of float/splat lowering in the aarch64 backend. Previously it was all living in Rust but to get to know it I translated it all to ISLE. I felt that the logic was a bit more understandable after the transition and more easily shows off what all the special cases are (the problem is more well-suited for ISLE I think than Rust, nothing against the prior code of course which was perfectly readable).