[X86] Merge adjacent stack adjustments in eliminateCallFramePseudoInstr (PR27140)

For code such as:

  void f(int, int);
  void g() {
      f(1, 2);
  }

compiled for 32-bit X86 Linux, Clang would previously generate:

  subl    $12, %esp
  subl    $8, %esp
  pushl   $2
  pushl   $1
  calll   f
  addl    $16, %esp
  addl    $12, %esp
  retl

This patch fixes that by merging adjacent stack adjustments in
eliminateCallFramePseudoInstr().

Differential Revision: http://reviews.llvm.org/D18627

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265039 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed