[ARM] Add lowering of uadd_sat to uq{add|sub}8 and uq{add|sub}16

This follow the lead of https://reviews.llvm.org/D68974 to add lowering
of unsigned saturated addition/subtraction.

Differential Revision: https://reviews.llvm.org/D105413
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failed leconte-build-gnu-7.5.0 #991728
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