Collect and coalesce DBG_VALUE instructions before emitting the function.

Correctly terminate the range of register DBG_VALUEs when the register is
clobbered or when the basic block ends.

The code is now ready to deal with variables that are sometimes in a register
and sometimes on the stack. We just need to teach emitDebugLoc to say 'stack
slot'.

llvm-svn: 128327