Add <imp-def> operands when reloading into physregs.

When an instruction only writes sub-registers, it is still necessary to
add an <imp-def> operand for the super-register.  When reloading into a
virtual register, rewriting will add the operand, but when loading
directly into a virtual register, the <imp-def> operand is still
necessary.

llvm-svn: 152095