[OpenCL] Simplify LLVM IR generated for OpenCL blocks

Summary:
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

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58388

llvm-svn: 354568