[DebugInfo] Handle stack slot offsets for spilled sub-registers in LDV

Summary:
Extend LDV so that stack slot offsets for spilled sub-registers
are added to the emitted debug locations. This is accomplished
by querying InstrInfo::getStackSlotRange().

With this change, LDV will add a DW_OP_plus_uconst operation to
the expression if a sub-register is spilled. Later on, PEI will
add an offset operation for the stack slot, meaning that we will
get expressions of the forms:

 * {DW_OP_constu #fp-offset, DW_OP_minus,
    DW_OP_plus_uconst #subreg-offset}

 * {DW_OP_plus_const #fp-offset,
    DW_OP_minus, DW_OP_plus_uconst #subreg-offset}

The two offset operations should ideally be merged.

Reviewers: rnk, aprantl, stoklund

Reviewed By: aprantl

Subscribers: dblaikie, bjope, nemanjai, JDevlieghere, llvm-commits

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D51612

llvm-svn: 341659