[OpenCL] Simplify LLVM IR generated for OpenCL blocks
Emit direct call of block invoke functions when possible, i.e. in case the
block is not passed as a function argument.
Also doing some refactoring of CodeGenFunction::EmitBlockCallExpr()
Reviewers: Anastasia, yaxunl, svenvh
Reviewed By: Anastasia
Differential Revision: https://reviews.llvm.org/D58388