[ARM][ConstantIslands] Fix stack mis-alignment caused by undoLRSpillRestore.

Authored by huihuiz on Mar 2 2020, 4:11 PM.


[ARM][ConstantIslands] Fix stack mis-alignment caused by undoLRSpillRestore.

It is not safe for ARMConstantIslands to undoLRSpillRestore. PrologEpilogInserter is
the one to ensure stack alignment, taking into consideration LR is spilled or not.

For noreturn function with StackAlignment 8 (function contains call/alloc),
undoLRSpillRestore cause stack be mis-aligned. Fixing stack alignment in
ARMConstantIslands doesn't give us much benefit, as undo LR spill/restore only
occur in large function with near branches only, also doesn't have callee-saved LR spill.

Reviewers: t.p.northover, rengolin, efriedma, apazos, samparker, ostannard

Reviewed By: ostannard

Subscribers: dmgreen, ostannard, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75288