[DebugInfo][FastISel] Fix dropping dbg.value()

Summary:
https://llvm.org/PR36263 shows that when compiling at -O0 a dbg.value()
instruction (that remains from an original dbg.declare()) is dropped
by FastISel. Since FastISel selects instructions by iterating a basic
block backwards, it drops the dbg.value if one of its operands is not
yet instantiated by a previously selected instruction.

Instead of calling 'lookUpRegForValue()' we can call 'getRegForValue()'
instead that will insert a placeholder for the operand to be filled in
when continuing the instruction selection.

Reviewers: aprantl, dblaikie, probinson

Reviewed By: aprantl

Subscribers: llvm-commits, dstenb, JDevlieghere

Differential Revision: https://reviews.llvm.org/D43386

llvm-svn: 325438