[PR66791][ARM] Replace __builtin_vext* with __buitlin_shuffle in vext intrinsics

[Prathamesh Kulkarni via Gcc-patches]

Hi Kyrill,
The attached patch replaces __builtin_vextv8qi with __builtin_shuffle
for vext_s8.
Just wanted to confirm if this is in the correct direction ?
If yes, I will send a follow up patch that converts for all vext intrinsics.

Thanks,
Prathamesh

vext-1.diff
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