[MIPS] Fix lowering of irregular vector arguments

The code deciding how to split the vector in register-sized integers used the integer division operator, thus rounding down the result.
Correct the computation for irregularly-sized types (non-power-of-two, non multiple of 8) by rounding the division result upwards.

Reviewed By: atanasyan

Differential Revision: https://reviews.llvm.org/D98189