This patch adds a canonicalization pattern for optimizing redundant
"pointer" fir.converts. Such converts prevent the StackArrays pass
to recognize fir.freemem for the corresponding fir.allocmem, e.g.:
```
%69 = fir.allocmem !fir.array<2xi32>
%71:2 = hlfir.declare %69(%70) {uniq_name = ".tmp.arrayctor"} :
(!fir.heap<!fir.array<2xi32>>, !fir.shape<1>) ->
(!fir.heap<!fir.array<2xi32>>, !fir.heap<!fir.array<2xi32>>)
%95 = fir.convert %71#1 :
(!fir.heap<!fir.array<2xi32>>) -> !fir.ref<!fir.array<2xi32>>
%100 = fir.convert %95 :
(!fir.ref<!fir.array<2xi32>>) -> !fir.heap<!fir.array<2xi32>>
fir.freemem %100 : !fir.heap<!fir.array<2xi32>>
```
I found this in `tonto`, but the change does not affect performance at all.
Anyway, it looks like a reasonable thing to do, and it makes easier
to compare the performance profiles with other compilers'.
2e637dbbb8