[SLPVectorizer] Relax alternate opcodes to accept any BinaryOperator pair

SLP currently only accepts (F)Add/(F)Sub alternate counterpart ops to be merged into an alternate shuffle.

This patch relaxes this to accept any pair of BinaryOperator opcodes instead, assuming the target's cost model accepts the vectorization+shuffle.

Differential Revision: https://reviews.llvm.org/D48477

llvm-svn: 335349