[mem2reg][debuginfo] Handle op_deref when converting dbg.declare

The conversion of dbg.declare into dbg.values doesn't take into account
the DIExpression attached to the intrinsic. In particular, when
converting:

```
store %val, ptr %alloca
dbg.declare(ptr %alloca, !SomeVar, !DIExpression())
```

Mem2Reg will try to figure out if `%val` has the size of `!SomeVar`. If
it does, then a non-undef dbg.value is inserted:

```
dbg.value(%val, !SomeVar, !DIExpression())
```

This makes sense: the alloca is _the_ address of the variable. So a
store to the alloca is a store to the variable. However, if the
expression in the original intrinsic is a `DW_OP_deref`, this logic is
not applicable:

```
store ptr %val, ptr %alloca
dbg.declare(ptr %alloca, !SomeVar, !DIExpression(DW_OP_deref))
```

Here, the alloca is *not* the address of the variable. A store to the
alloca is *not* a store to the variable. As such, querying whether
`%val` has the same size as `!SomeVar` is meaningless.

This patch addresses the issue by:
1. Allowing the conversion when the expression is _only_ a `DW_OP_deref`
without any other expressions (see code comment).
2. Checking that the expression does not start with a `DW_OP_deref`
before applying the logic that checks whether the value being stored and
the variable have the same length.

Differential Revision: https://reviews.llvm.org/D142160