[mono] Share large fallback immediate-parameter workaround code sequences

[imhameed]

Mostly a note to myself. Several SIMD instructions on amd64 (e.g. shuffles, dot products, blending, etc.) take immediate control masks. We currently generate large branching fallback code inline but this is not great for code size. Can use llvm.is.constant to keep the large materialized branching code sequence out of line while still allowing constant propagation to simplify shuffles with control masks that happen to be constant after optimization.

[EgorBo]

LLVM needs a sort of llvm.convert_to_const(value, range) so it can generate a switch itself