[lldb] [lit] Use constexpr and better constraints in Register tests

Use constexpr to explicitly indicate that we're dealing with integer
constants, and provoke clang to assign them straight to registers
whenever possible.  Adjust input constraints in %mmN tests to "rm"
as using integer constants is apparently disallowed there.  Also
use "i" for %rN tests, as we don't want clang to accidentally clobber
those general purpose registers while assigning to them (however
unlikely that is).

llvm-svn: 359228