[RISCV][GISel] Add isel patterns for i16 load/store (#116293)

In order to support f16 load/store we need to make load/stores with s16
register type legal. If regbank selection doesn't pick the FPR bank,
we'll be left with a GPR load or store which we don't have isel patterns
for from SelectionDAG.

In order to add the patterns we need to make i16 a legal type for the
GPR register class.

Tests are currently disabling the legality check because I haven't
update the legalizer yet.

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