[clang] Improve handling of physical registers in inline assembly operands.

Change EmitAsmStmt() to

- Not tie physregs with the "+r" constraint, but instead add the hard
  register as an input constraint. This makes "+r" and "=r":"r" look the same
  in the output.

  Background: Macro intensive user code may contain inline assembly
  statements with multiple operands constrained to the same physreg. Such a
  case (with the operand constraints "+r" : "r") currently triggers the
  TwoAddressInstructionPass assertion against any extra use of a tied
  register. Furthermore, TwoAddress will insert a COPY to that physreg even
  though isel has already done so (for the non-tied use), which may lead to a
  second redundant instruction currently. A simple fix for this is to not
  emit tied physreg uses in the first place for the "+r" constraint, which is
  what this patch does.

- Give an error on multiple outputs to the same physical register.

  This should be reported and this is also what GCC does.

Review: Ulrich Weigand, Aaron Ballman, Jennifer Yu, Craig Topper

Differential Revision: https://reviews.llvm.org/D87279