[flang] Passing vector type to MERGE causing UNSUPPORTED type assertion

Consider the following code:

```fortran
program vec_fir_box_ice
vector(integer*4) va(1), vb(1)
integer*4 sa(2), sb(2)
equivalence(va, sa)
equivalence(vb, sb)
logical mask(1)

mask(1) = .true.
sa = (/ 1, 2 /)
sb = (/ -1, -2 /)

! merge() on vector() array: at -O0 this produces a fir.box with
! mlir::VectorType element type, triggering the ICE.
vb = merge(va, vb, mask)

print *, 'sb(1)=', sb(1), '(expected 1)'
print *, 'sb(2)=', sb(2), '(expected 2)'
end
```

Flang currently outputs:

```
unsupported type
UNREACHABLE executed at flang/lib/Optimizer/Dialect/FIRType.cpp:589!
```

Expected output:

```
sb(1)= 1 (expected 1)
sb(2)= 2 (expected 2)
```