This is trying to fix one of the potential regressions seen in D58521, but it's a problem independent of that patch as shown in the diffs here.
We are aggressively converting 'anyext' to 'zext' in isel to avoid partial reg stalls, but that shouldn't be a problem for pinsrb because the instruction only uses the low byte of the 32-bit scalar reg. AFAICT, this isn't a problem for pinsrw because we promote all 16-bit ops.
I'm not sure why we don't get load folding with some of the fast-isel tests, but I assume that's as expected or an independent problem.