Revert "Use SIMD intrinsics for vector shifts"

[RalfJung]

Reverts #1955 to fix rust-lang/rust#159831 and to fix rust-lang/rust#159801.
Cc @sayantn

No tests because the per-intrinsic codegen is actually fine, it's the combined codegen with other operations where things fall apart.

[rustbot]

r? @adamgemmell

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[RalfJung]

(CI seems to be generally broken, that's not from my PR.)