[DAGCombiner] refactor select-of-FP-constants transform

This transform needs to be limited. 

We are converting to a constant pool load very early, and we 
are turning loads that are independent of the select condition 
(and therefore speculatable) into a dependent non-speculatable 
load.

We may also be transferring a condition code from an FP register
to integer to create that dependent load.

llvm-svn: 347424