BumpPtrAllocator: don't accept 0 for the alignment parameter

It seems unnecessary to have to use an extra branch to check for this special case.

http://reviews.llvm.org/D4945

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216036 91177308-0d34-0410-b5e6-96231b3b80d8
InstCombine: Annotate sub with nsw when we prove it's safe

We can prove that a 'sub' can be a 'sub nsw' under certain conditions:
- The sign bits of the operands is the same.
- Both operands have more than 1 sign bit.

The subtraction cannot be a signed overflow in either case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216037 91177308-0d34-0410-b5e6-96231b3b80d8