[MIPS GlobalISel] Select any extending load and truncating store

Make behavior of G_LOAD in widenScalar same as for G_ZEXTLOAD and
G_SEXTLOAD. That is perform widenScalarDst to size given by the target
and avoid additional checks in common code. Targets can reorder or add
additional rules in LegalizeRuleSet for the opcode to achieve desired
behavior.

Select extending load that does not have specified type of extension
into zero extending load.

Select truncating store that stores number of bytes indicated by size
in MachineMemoperand.

Differential Revision: https://reviews.llvm.org/D57454

llvm-svn: 353520