It can happen that a live debug variable is the last use of a sub-register, and
the register allocator will pick a larger register class for the virtual
register.  If the allocated register doesn't support the sub-register index,
just use %noreg for the debug variables instead of asserting.

In PR9872, a debug variable ends up in the sub_8bit_hi part of a GR32_ABCD
register. The register is split and one part is inflated to GR32 and assigned
%ESI because there are no more normal uses of sub_8bit_hi.

Since %ESI doesn't have that sub-register, substPhysReg asserted. Now it will
simply insert a %noreg instead, and the debug variable will be marked
unavailable in that range.

We don't currently have a way of saying: !"value" is in bits 8-15 of %ESI, I
don't know if DWARF even supports that.

llvm-svn: 131073