[MIPS GlobalISel] Lower i64 arguments

Lower integer arguments larger then 32 bits for MIPS32.
setMostSignificantFirst is used in order for G_UNMERGE_VALUES and
G_MERGE_VALUES to always hold registers in same order, regardless of
endianness.

Patch by Petar Avramovic.

Differential Revision: https://reviews.llvm.org/D52409

llvm-svn: 343315