Fixing truncate. Previously we were emitting truncate from r16 to r8 as

Fixing truncate. Previously we were emitting truncate from r16 to r8 as
movw. That is we promote the destination operand to r16. So
        %CH = TRUNC_R16_R8 %BP
is emitted as
        movw %bp, %cx.

This is incorrect. If %cl is live, it would be clobbered.
Ideally we want to do the opposite, that is emitted it as
        movb ??, %ch
But this is not possible since %bp does not have a r8 sub-register.

We are now defining a new register class R16_ which is a subclass of R16
containing only those 16-bit registers that have r8 sub-registers (i.e.
AX - DX). We isel the truncate to two instructions, a MOV16to16_ to copy the
value to the R16_ class, followed by a TRUNC_R16_R8.

Due to bug 770, the register colaescer is not going to coalesce between R16 and
R16_. That will be fixed later so we can eliminate the MOV16to16_. Right now, it
can only be eliminated if we are lucky that source and destination registers are
the same.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28164 91177308-0d34-0410-b5e6-96231b3b80d8