[mlir][memref] Fix num elements in lowering of memref.alloca op to LLVM

Fixes a mistake in the lowering of memref.alloca to llvm.alloca, as llvm.alloca uses the number of elements to allocate in the stack and not the size in bytes.

Reference:
LLVM IR: https://llvm.org/docs/LangRef.html#alloca-instruction
LLVM MLIR: https://mlir.llvm.org/docs/Dialects/LLVM/#llvmalloca-mlirllvmallocaop

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D150705