[alpha] cleanup sign_extension patterns

[Richard Henderson <rth at redhat dot com>]

We don't get a chance to use attribute enabled here, but we can
eliminate some unnecessary expanders.

The non-BWX expansion paths wound up subregging the input to 
DImode to perform the operation and subregging it back.  Exactly
as the generic code paths would do.

I didn't investigate what eliminating the sub-word patterns
for BWX code has on the generated code.  In theory these are
not needed either, also handled by the generic code paths...


r~

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