[OpenMP][IRBuilder] Handle floats for atomic update and fix AllocaIP for update/capture

This patch fixes `createAtomicUpdate` for lowering with float types.
Test added for the same.

This patch also changes the alloca argument for createAtomicUpdate and
createAtomicCapture from `Instruction*` to `InsertPointTy`. This is in
line with the other functions of the OpenMPIRBuilder class which take
AllocaIP as an `InsertPointTy`.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D118227