Do better with physical reg operands (typically, from inline asm)

Do better with physical reg operands (typically, from inline asm)
in local register allocator.  If a reg-reg copy has a phys reg
input and a virt reg output, and this is the last use of the phys
reg, assign the phys reg to the virt reg.  If a reg-reg copy has
a phys reg output and we need to reload its spilled input, reload
it directly into the phys reg than passing it through another reg.

Following 76208, there is sometimes no dependency between the def of
a phys reg and its use; this creates a window where that phys reg
can be used for spilling (this is true in linear scan also).  This
is bad and needs to be fixed a better way, although 76208 works too
well in practice to be reverted.  However, there should normally be
no spilling within inline asm blocks.  The patch here goes a long way
towards making this actually be true.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@91485 91177308-0d34-0410-b5e6-96231b3b80d8