[llvm] [AArch64][GlobalISel] Legalize s16 G_FCONSTANT to avoid widening to G_CONSTANT (PR #161205)

[Ryan Cowan via llvm-commits]

HolyMolyCowMan wrote:

By moving the `matchFConstantToConstant` to the post-legalizer lowering, patches such as https://github.com/llvm/llvm-project/pull/160902 can more easily fold operations involving `G_FCONSTANT`s, and still make use of zero registers when applicable.

https://github.com/llvm/llvm-project/pull/161205