[MIPS] Implement support for -mstack-alignment.

Summary:
This is modeled on the implementation for x86 which stores the command line
option in a 'StackAlignOverride' field in MipsSubtarget and then uses this
to compute a 'stackAlignment' value in
MipsSubtarget::initializeSubtargetDependencies.

The stackAlignment() method in MipsSubTarget is renamed to getStackAlignment()
and returns the computed 'stackAlignment'.

Reviewers: sdardis

Reviewed By: sdardis

Subscribers: llvm-commits, arichardson

Differential Revision: https://reviews.llvm.org/D35874

llvm-svn: 310891
diff --git a/llvm/lib/Target/Mips/MipsSEFrameLowering.cpp b/llvm/lib/Target/Mips/MipsSEFrameLowering.cpp
index 66afe99..4c7e25b 100644
--- a/llvm/lib/Target/Mips/MipsSEFrameLowering.cpp
+++ b/llvm/lib/Target/Mips/MipsSEFrameLowering.cpp
@@ -390,7 +390,7 @@
 }
 
 MipsSEFrameLowering::MipsSEFrameLowering(const MipsSubtarget &STI)
-    : MipsFrameLowering(STI, STI.stackAlignment()) {}
+    : MipsFrameLowering(STI, STI.getStackAlignment()) {}
 
 void MipsSEFrameLowering::emitPrologue(MachineFunction &MF,
                                        MachineBasicBlock &MBB) const {