Reapply r183552. This time, use a standard type for the option to avoid template
instantiation issue with non-standard type.

Add a backend option to warn on a given stack size limit.
Option: -mllvm -warn-stack-size=<limit>
Output (if limit is exceeded):
warning: Stack size limit exceeded (<actual size>) in <functionName>.

The longer term plan is to hook that to a clang warning.
PR:4072
<rdar://problem/13987214>.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183595 91177308-0d34-0410-b5e6-96231b3b80d8
[mips] Use a helper function which compares the size of the source and
destination operands of an instruction.

No functionality changes.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183596 91177308-0d34-0410-b5e6-96231b3b80d8
Fix unused variable warning from my previous patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183601 91177308-0d34-0410-b5e6-96231b3b80d8