Recommit r231324 with a fix to the ARM execution domain code

Recommit r231324 with a fix to the ARM execution domain code
to disable lane switching if we don't actually have the instruction
set we want to switch to. Models the earlier check above the
conditional for the pass.

The testcase is one that triggered with the assert that's added
as part of the fix, use it to avoid adding a new testcase as it
highlights the same problem.

llvm-svn: 231539