[Mosaic GPU] Execute relayouts in the lowering of `layout_cast`.
As I was rewriting Pallas tests, I noticed a case where we essentially would
generate code with the form:
```
x = create() # out_layout: strided layout
y = layout_cast x # {in,out}_layout = WGMMA layout
z = consume(y) # in_layout: strided layout
```
which would end up silently succeeding. Since the intent is for `layout_cast`
to be used as a user annotation to guide layout inference, we should make
sure to crash in such cases---when a bad relayout is involved, and the layout
we intended to use is not propagated.
PiperOrigin-RevId: 773592922
