[CodeGen] Emit Constants for immediate inlineasm arguments.

For inline assembly immediate constraints, we currently always use
EmitScalarExpr, instead of directly emitting the constant. When the
overflow sanitizer is enabled, this generates overflow intrinsics
instead of constants.

Instead, emit a constant for constraints that either require an
immediate (e.g. 'I' on X86), or only accepts constants (immediate
or symbolic; i.e., don't accept registers or memory).

Fixes PR19763.

Differential Revision: http://reviews.llvm.org/D10255

llvm-svn: 239549