swift234

[let-properies-opt] Fix a bug which where a let property was wrongly considered to have a constant value.

The optimization pass was inspecting only init methods to determine if a given let property is defined
in the same way by all initializers. But this is not enough in certain cases, e.g. when some of the
initializers were inlined into the application code and the body of the inlined SIL function representing
such an initializer was removed afterwards by the dead function elimination pass. In such situations,
the Let Properties Optimization pass was assuming that there is only one initializer and considered the
constant let property value defined there as the only possible value of this let property. Therefore it
propagated it into let-property uses, which resulted in an incorrect code.

The right thing to do is to analyze all assignments to a given let property whether they are inside initializer
SIL functions or not. This makes sure that all possible values of a let property are analyzed and compared.
The propagation of a constant let property value can only happen if all found possible values are all the same.

Fixes SR-1026 and rdar://25303106