Salvage debug info from instructions about to be deleted

Salvage debug info from instructions about to be deleted

This patch improves debug info quality in InstCombine by looking at
values that are about to be deleted, checking whether there are any
dbg.value instrinsics referring to them, and potentially encoding the
semantics of the deleted instruction into the dbg.value's
DIExpression.

In the example in the testcase (which was extracted from XNU) there is a sequence of

  %4 = load %struct.entry*, %struct.entry** %next2, align 8, !dbg !41
  %5 = bitcast %struct.entry* %4 to i8*, !dbg !42
  %add.ptr4 = getelementptr inbounds i8, i8* %5, i64 -8, !dbg !43
  %6 = bitcast i8* %add.ptr4 to %struct.entry*, !dbg !44
  call void @llvm.dbg.value(metadata %struct.entry* %6, i64 0, metadata !20, metadata !21), !dbg 34

When these instructions are eliminated by instcombine one after
another, we can still salvage the otherwise dead debug info:

- Bitcasts have no effect, so have the dbg.value point to operand(0)
- Loads can be expressed via a DW_OP_deref
- Constant gep instructions can be replaced by DWARF expression arithmetic

The API introduced by this patch is not specific to instcombine and
can be useful in other places, too.

rdar://problem/30725338

Differential Revision: https://reviews.llvm.org/D30919

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297971 91177308-0d34-0410-b5e6-96231b3b80d8