[x86] split tests for FP and integer horizontal math

These are similar patterns, but when you throw AVX512 onto the pile,
the number of variations explodes. For FP, we really don't care about
AVX1 vs. AVX2 for FP ops. There may be some superficial shuffle diffs,
but that's not what we're testing for here, so I removed those RUNs.

Separating by type also lets us specify 'sse3' for the FP file vs. 'ssse3'
for the integer file...because x86.

llvm-svn: 350357