[selectiondag] Check PromotedFloats map during expansive checks.

Summary:
`PromotedFloats` needs to be checked in 
`DAGTypeLegalizer::PerformExpensiveChecks`. This patch fixes a few type
legalization failures with expansive checks for ARM fp16 tests.

Reviewers: baldrick, bogner, arsenm

Subscribers: arsenm, aemerson, llvm-commits

Differential Revision: https://reviews.llvm.org/D28187

llvm-svn: 290796