[GlobalISel] Improve stack slot tracking in dbg.values

For IR like:

```
%alloca = alloca ...
dbg.value(%alloca, !myvar, OP_deref(<other_ops>))
```

GlobalISel lowers it to MIR:

```
%some_reg = G_FRAME_INDEX <stack_slot>
DBG_VALUE %some_reg, !myvar, OP_deref(<other_ops>)
```

In other words, if the value of `!myvar` can be obtained by
dereferencing an alloca, in MIR we say that the _location_ of a variable
is obtained by dereferencing register %some_reg (plus some
`<other_ops>`).

We can instead remove the use of `%some_reg`: the location of `!myvar`
_is_ `<stack_slot>` (plus some `<other_ops>`). This patch implements
this transformation, which improves debug information handling in O0, as
these registers hardly ever survive register allocation.

A note about testing: similar to what was done in D76934
(f24e2e9e), this patch exposed a bug in the Builder class when
using `-debug`, where we tried to print an incomplete instruction. The
changes in `MachineIRBuilder.cpp` address that.

Differential Revision: https://reviews.llvm.org/D147536