[mlir] Remove over specified memory effects

The standard and gpu dialect both have `alloc` operations which use the
memory effect `MemAlloc`.  In both cases, it is specified on both  the
operation itself and on the result.  This results in two memory effects
being created for these operations.  When `MemAlloc` is defined on an
operation, it represents some background effect which the compiler
cannot reason about, and  inhibits the ability of the compiler to
remove dead `std.alloc` operations.  This change removes the uneeded
`MemAlloc` effect from these operations and leaves the effect on the
result, which allows dead allocs to be erased.

There is the same problem, but to a lesser extent, with MemFree, MemRead
and MemWrite. Over-specifying these traits is not currently inhibiting
any optimization.

Differential Revision: https://reviews.llvm.org/D94662