[MLIR] Introduce std.alloca op

[MLIR] Introduce std.alloca op

Introduce the alloca op for stack memory allocation. When converting to the
LLVM dialect, this is lowered to an llvm.alloca. Refactor the std to
llvm conversion for alloc op to reuse with alloca. Drop useAlloca option
with alloc op lowering.

Differential Revision: https://reviews.llvm.org/D76602