Put VMOVS widening under a command line option, off by default.

It appears that our use of the imp-use and imp-def flags with
sub-registers is not yet robust enough to support this.

The failing test case is complicated, I am working on a reduction.

<rdar://problem/10044201>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@138861 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed