… update API

Rather than adding the use-tracking to llir/llvm/ir, we currently add
this to irutil so we may get more experience with the API and see if its
the right fit for instruction and terminator operand use-tracking and
value update.

Note: this is the stronger version of the API, as outlined by @pwaller
in llir/llvm#42 (comment)

Notably, operands are tracked by pointer (*value.Value) which also enables
updating values using the same API.

Fixes llir/llvm#42.

…tUses

Note: we should try to arrive at a more general way of handling custom
user-defined instructions.

As suggested by @pwaller in llir#1 (comment)

…ue operands

Without this change, repeated invokations of InstUses (as used in FuncUses)
would result in incorrect results.

	for _, inst := range block.Insts {
		backing = InstUses(backing, inst)
	}

Since we already track a pointer to the used value in the Use structure,
we can pass Use by value. This should help reduce memory allocations.

…is merged

Note: this implementation of dce is a toy example. It does not take deep
value uses into account, and as such, cannot handle use tracking when
local variables are used within constant expressions.

This issue would be resolved when irutil.FuncUses is extended to handle
deep value use tracking.