[llvm] Move CallInst::CreateMalloc to IRBuilderBase::CreateMalloc

This removes `CreateMalloc` from `CallInst` and adds it to the `IRBuilderBase`
class.

We no longer needed the `Instruction *InsertBefore` and
`BasicBlock *InsertAtEnd` arguments of the `createMalloc` helper
function because we're using `IRBuilder` now. That's why I we also don't
need 4 `CreateMalloc` functions, but only two.

Differential Revision: https://reviews.llvm.org/D158861