[ARM] Reject CSEL instructions with invalid operands

Summary:
According to the Armv8.1-M manual CSEL, CSINC, CSINV and CSNEG are
"constrained unpredictable" when SP is used as the source register Rn.

The assembler should diagnose this case.

Reviewers: momchil.velikov, dmgreen, ostannard, simon_tatham, t.p.northover

Reviewed By: ostannard

Subscribers: javed.absar, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65505

llvm-svn: 367433