register is available and when it's profitable.

e.g.
     xorq  %r12<kill>, %r13
     addq  %rax, -184(%rbp)
     addq  %r13, -184(%rbp)
==>
     xorq  %r12<kill>, %r13
     movq  -184(%rbp), %r12
     addq  %rax, %r12
     addq  %r13, %r12
     movq  %r12, -184(%rbp)

Two more instructions, but fewer memory accesses. It can also open up
opportunities for more optimizations.

llvm-svn: 69341

llvm-svn: 68099

llvm-svn: 68092

Spiller may unfold load / mod / store instructions as an optimization when the would be loaded value is available in a register. It needs to check if it's legal to clobber the register. Also, the register can contain values of multiple spill slots, make sure to check all instead of just the one being unfolded.

llvm-svn: 67068

llvm-svn: 66780

Reorganization: Move the Spiller out of VirtRegMap.cpp into its own files.  No (intended) functionality change.

llvm-svn: 66720