[stackmaps] Start legalizing live variable operands

Prior to this change, live variable operands passed to
`llvm.experimental.stackmap` would be emitted directly to target nodes,
meaning that they don't get legalised. The upshot of this is that LLVM
may crash when encountering illegally typed target nodes.

e.g. https://github.com/llvm/llvm-project/issues/21657

This change introduces a platform independent stackmap DAG node whose
operands are legalised as per usual, thus avoiding aforementioned
crashes.

Note that some kinds of argument are still not handled properly, namely
vectors, structs, and large integers, like i128s. These will need to be
addressed in follow-up changes.

Note also that this does not change the behaviour of
`llvm.experimental.patchpoint`. A follow up change will do the same for
this intrinsic.

Differential review:
https://reviews.llvm.org/D125680