[VE] Change displacement type in MEM..i from i32 to i64

In selection patterns, addresses (like tblockaddr) are passed as the
displacement (the i in MEM..i) to instructions taking MEM operands.
Since addresses are 64-bit, having this part of the MEM..i operand as
i32 causes a type inference error. The instructions actually only encode
32 bits of the displacement, but there is no way to manually extract
these bits (either the high or the low half) in selection patterns.

This didn't happen before, because of a bug in type inference when
dealing with iPTR.