[Pipeliner] Pipeliner should mark physical registers as used

The software pipeliner attempts to delete dead instructions after
generating the pipelined loop. The code looks for uses of each 
instruction. Physical registers should be treated differently because
the use chains do not exist. The code that checks for dead 
instructions should assume that definitions of physical registers
are used if the operand doesn't contain the dead flag.

Patch by Brendon Cahoon.

llvm-svn: 328509