Unify the way we report overflow in increment/decrement operator.

Summary:
Make sure signed overflow in "x--" is checked with
llvm.ssub.with.overflow intrinsic and is reported as:
  "-2147483648 - 1 cannot be represented in type 'int'"
instead of:
  "-2147483648 + -1 cannot be represented in type 'int'"
, like we do for unsigned overflow.

Test Plan: clang + compiler-rt regression test suite

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D8236

llvm-svn: 235568