[SelectionDAG] treat X constrained labels as i for asm

Completely rework how we handle X constrained labels for inline asm.

X should really be treated as i. Then existing tests can be moved to use
i D115410 and clang can just emit i D115311. (D115410 and D115311 are
callbr, but this can be done for label inputs, too).

Coincidentally, this simplification solves an ICE uncovered by D87279
based on assumptions made during D69868.

This is the third approach considered. See also discussions v1 (D114895)
and v2 (D115409).

Reported-by: kernel test robot <lkp@intel.com>
Fixes: https://github.com/ClangBuiltLinux/linux/issues/1512

Reviewed By: void, jyknight

Differential Revision: https://reviews.llvm.org/D115688