Mapping of FP operations to constrained intrinsics

A new function 'getConstrainedIntrinsic' is added, which for any gived
instruction returns id of the corresponding constrained intrinsic. If
there is no constrained counterpart for the instruction or the instruction
is already a constrained intrinsic, the function returns zero.

This is recommit of 115b3ace, reverted in
8160dd58.

Differential Revision: https://reviews.llvm.org/D69562