[mlir][Vector] Add fastmath flags to vector.reduction (#66905)

This revision pipes the fastmath attribute support through the
vector.reduction op. This seemingly simple first step already requires
quite some genuflexions, file and builder reorganization. In the
process, retire the boolean reassoc flag deep in the LLVM dialect
builders and just use the fastmath attribute.

During conversions, templated builders for predicated intrinsics are
partially cleaned up. In the future, to finalize the cleanups, one
should consider adding fastmath to the VPIntrinsic ops.