[InstCombine] Enable more reassociations using FMF 'reassoc' + 'nsz'

Reassociation of math ops in some contexts (especially vector contexts)
has generally only been happening when the 'fast' FMF was set.  This
enables reassoication when only the finer grained controls 'reassoc' and
'nsz' are set.

Differential Revision: https://reviews.llvm.org/D47335

llvm-svn: 333221