[Assignment Tracking] Fix migrateDebuginfo in SROA

Without this patch, migrateDebugInfo doesn't understand how to handle existing
fragments that are smaller than the to-be-split store. This can occur
if. e.g. a vector store (1 dbg.assign) is split (many dbg.assigns - 1 fragment
for each scalar) and later those stores are re-vectorized (many dbg.assigns),
and then SROA runs on that.

The approach taken in this patch is to drop intrinsics with fragments outside
of the slice.

For example, starting with:

  store <2 x float> %v, ptr %dest !DIAssignID !1
  call void @llvm.dbg.assign(..., DIExpression(DW_OP_LLVM_fragment, 0, 32), !1, ...)
  call void @llvm.dbg.assign(..., DIExpression(DW_OP_LLVM_fragment, 32, 32), !1, ...)

When visiting the slice of bits 0 to 31 we get:

  store float %v.extract.0, ptr %dest !DIAssignID !2
  call void @llvm.dbg.assign(..., DIExpression(DW_OP_LLVM_fragment, 0, 32), !2, ...)

The other dbg.assign associated with the currently-split store is dropped for
this split part. And visiting bits 32 to 63 we get the following:

  store float %v.extract.1, ptr %adjusted.dest !DIAssignID !3
  call void @llvm.dbg.assign(..., DIExpression(DW_OP_LLVM_fragment, 32, 32), !3, ...)

I've added two tests that cover this case.

Implementing this meant re-writing the fragment-calculation part of
migrateDebugInfo to work with the absolute offset of the new slice in terms of
the base alloca (instead of the offset of the slice into the new alloca), the
fragment (if any) of the variable associated with the base alloca, and the
fragment associated with the split store. Because we need the offset into the
base alloca for the variables being split, some careful wiring is required for
memory intrinsics due to the fact that memory intrinsics can be split when
either the source or dest allocas are split. In the case where the source
alloca drives the splitting, we need to be careful to pass migrateDebugInfo the
information in relation to the dest alloca.

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D143146