[ISel] Fix DAG divergence after new FMA combine

[ISel] Fix DAG divergence after new FMA combine

D132837 introduced a new DAG combine that used MorphNodeTo to morph an
FMUL into an FMA. It turns out that MorphNodeTo does not properly update
the divergence bit for users of the morphed node, causing an assertion
failure on the new test case:

llc: SelectionDAG.cpp:10486: void llvm::SelectionDAG::VerifyDAGDivergence(): Assertion `calculateDivergence(N) == N->isDivergent() && "Divergence bit inconsistency detected"' failed.

Fixing MorphNodeTo to propagate the divergence bit is tricky because of
the way it is used to select machine instructions, so use getNode and
ReplaceAllUsesOfValueWith instead.

Differential Revision: https://reviews.llvm.org/D134810