SILOptimizer: a new optimization for copy-on-write
Constant folds the uniqueness result of begin_cow_mutation instructions, if it can be proved that the buffer argument is uniquely referenced.
For example:
%buffer = end_cow_mutation %mutable_buffer
// ...
// %buffer does not escape here
// ...
(%is_unique, %mutable_buffer2) = begin_cow_mutation %buffer
cond_br %is_unique, ...
is replaced with
%buffer = end_cow_mutation [keep_unique] %mutable_buffer
// ...
(%not_used, %mutable_buffer2) = begin_cow_mutation %buffer
%true = integer_literal 1
cond_br %true, ...
Note that the keep_unique flag is set on the end_cow_mutation because the code now relies on that the buffer is really uniquely referenced.
The optimization can also handle def-use chains between end_cow_mutation and begin_cow_mutation which involve phi-arguments.
An additional peephole optimization is performed: if the begin_cow_mutation is the only use of the end_cow_mutation, the whole pair of instructions is eliminated.
