Rename the VECREDUCE_STRICT_{FADD,FMUL} SDNodes to VECREDUCE_SEQ_{FADD,FMUL}.

The STRICT was causing unnecessary confusion. I think SEQ is a more accurate
name for what they actually do, and the other obvious option of "ORDERED"
has the issue of already having a meaning in FP contexts.

Differential Revision: https://reviews.llvm.org/D88791