Reapply ARM: Do not spill CSR to stack on entry to noreturn functions

Reapply ARM: Do not spill CSR to stack on entry to noreturn functions

Should fix UBSan bot by also checking there's no "uwtable" attribute
before skipping. Otherwise the unwind table will be useless since its
moves expect CSRs to actually be preserved.

A noreturn nounwind function can be expected to never return in any way, and by
never returning it will also never have to restore any callee-saved registers
for its caller. This makes it possible to skip spills of those registers during
function entry, saving some stack space and time in the process. This is rather
useful for embedded targets with limited stack space.

Should fix PR9970.

Patch mostly by myeisha (pmb).

llvm-svn: 329494