Optimizer: add simplification for fix_lifetime
Canonicalize a `fix_lifetime` from an address to a `load` + `fix_lifetime`:
```
%1 = alloc_stack $T
...
fix_lifetime %1
```
->
```
%1 = alloc_stack $T
...
%2 = load %1
fix_lifetime %2
```
This transformation is done for `alloc_stack` and `store_borrow` (which always has an `alloc_stack` operand).
The benefit of this transformation is that it enables other optimizations, like mem2reg.
This peephole optimization was already done in SILCombine, but it didn't handle store_borrow.
A good opportunity to make an instruction simplification out of it.
This is part of fixing regressions when enabling OSSA modules:
rdar://140229560
