AltiVec support toasted in HEAD

[Joern Rennecke]

zero_constant just matches CONST0_RTX(V8HImode), so if for some reason
whomever created the 0 const_vector did not use const_tiny_rtx[0][V8HImode]
for the zero constant, the pattern won't match.

Who is responsible for using const_tiny_rtx, gen_rtx_CONST_VECTOR,
or the caller of gen_rtx_CONST_VECTOR ?

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