[SVE][Inline-Asm] Fix -Wimplicit-fallthrough in AArch64ISelLowering.cpp

Summary: Adds break to 'x' case in getRegForInlineAsmConstraint added by D66302, fixing the unintentional fallthrough.

Reviewers: sdesmalen, rovka, cameron.mcinally, greened, gribozavr, ruiu

Reviewed By: sdesmalen

Subscribers: bjope, javed.absar, tschuett, kristof.beyls, rkruppe, psnobl, llvm-commits, cfe-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67095

llvm-svn: 370769