Fix inline assembler constraint validation

The current constraint logic is both too lax and too strict. It fails
for input outside the [INT_MIN..INT_MAX] range, but it also implicitly
accepts 0 as value when it should not. Adjust logic to handle both
correctly.

Differential Revision: https://reviews.llvm.org/D58649


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@354937 91177308-0d34-0410-b5e6-96231b3b80d8