Reapply "[SelectionDAG] Selection of DBG_VALUE using a PHI node result (pt 2)"

Reapply "[SelectionDAG] Selection of DBG_VALUE using a PHI node result (pt 2)"

Summary:
This reverts SVN r331441 (reapplies r331337), together with a fix
in to handle an already existing fragment expression in the
dbg.value that must be fragmented due to a split PHI node.

This should solve the problem seen in PR37321, which was the
reason for the revert of r331337.

The situation in PR37321 is that we have a PHI node like this

   %u.sroa = phi i80 [ %u.sroa.x, %if.x ],
                     [ %u.sroa.y, %if.y ],
                     [ %u.sroa.z, %if.z ]

and a dbg.value like this

  call void @llvm.dbg.value(metadata i80 %u.sroa,
                            metadata !13,
                            metadata !DIExpression(DW_OP_LLVM_fragment, 0, 80))

The phi node is split into three 32-bit PHI nodes

  %30:gr32 = PHI %11:gr32, %bb.4, %14:gr32, %bb.5, %27:gr32, %bb.8
  %31:gr32 = PHI %12:gr32, %bb.4, %15:gr32, %bb.5, %28:gr32, %bb.8
  %32:gr32 = PHI %13:gr32, %bb.4, %16:gr32, %bb.5, %29:gr32, %bb.8

but since the original value only is 80 bits we need to adjust the size
of the last fragment expression, and with this patch we get

  DBG_VALUE debug-use %30:gr32, debug-use $noreg, !"u", !DIExpression(DW_OP_LLVM_fragment, 0, 32)
  DBG_VALUE debug-use %31:gr32, debug-use $noreg, !"u", !DIExpression(DW_OP_LLVM_fragment, 32, 32)
  DBG_VALUE debug-use %32:gr32, debug-use $noreg, !"u", !DIExpression(DW_OP_LLVM_fragment, 64, 16)

Reviewers: vsk, aprantl, mstorsjo

Reviewed By: aprantl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46384

llvm-svn: 331464