revert r325515: [TTI CostModel] change default cost of FP ops to 1 (PR36280)

revert r325515: [TTI CostModel] change default cost of FP ops to 1 (PR36280)

There are too many perf regressions resulting from this, so we need to
investigate (and add tests for) targets like ARM and AArch64 before
trying to reinstate.

llvm-svn: 325658