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

Summary:
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