[mlir][MemRef] Fix MemRefCastOpLowering for 32 bit index type.

The lowering creates llvm.insertvalue with the rank value, so it needs to use
index type instead of 64 bit integer type. Otherwise, we get an error:

llvm.insertvalue' op Type mismatch: cannot insert 'i64' into '!llvm.struct<(i32, ptr<i8>)>'

Differential Revision: https://reviews.llvm.org/D119534