[InstCombine] Make `FoldOpIntoSelect` handle non-constants and use condition to deduce constants.

[goldstein.w.n]

Summary

1. Make FoldOpIntoSelect handle non-constants by returning nullptr instead of aborting.
2. Make the fold use the information present in the condition for deducing constants i.e: ` %c = icmp eq i8 %x, 10 %s = select i1 %c, i8 3, i8 2 %r = mul i8 %x, %s `
   
   If we fold the mul into the select, on the true side we insert 10 for %x in the mul.

[goldstein.w.n]

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[goldstein.w.n]

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