[MIPS GlobalISel] Select addiu

Introduce IntImmLeaf version of PatLeaf immSExt16 for 32-bit immediates.
Change immSExt16 with imm32SExt16 for addiu.
This keeps same behavior for SDAG and allows for GlobalISel selectImpl
to select 'G_CONSTANT imm' + G_ADD into ADDIu when 32-bit imm satisfies
imm32SExt16 predicate: sign extending 16 low bits of imm is equal to imm.

Differential Revision: https://reviews.llvm.org/D70184