Match types of accumulator and result for llvm.experimental.vector.reduce.fadd/fmul

The scalar start/accumulator value of the fadd- and fmul reduction
should match the result type of the reduction, as well as the vector
element-type of the input vector. Although this was not explicitly
specified in the LangRef, it was taken for granted in code implementing
the reductions. The patch also fixes the LangRef by adding this
constraint.

Reviewed By: aemerson, nikic

Differential Revision: https://reviews.llvm.org/D60260

llvm-svn: 361133