swift234

Reuse type metadata in the most obvious cases.

This avoids redundant calls of swift_get*Metadata() if the metadata is reused in the same basic block
or if it is already availabe in the function entry block.
There is still room for improvement, e.g. by not limiting the reuse to a single block.

This change reduces the dylib codesize by about 11%, mostly in protocoll witnesses (-34%).

It also results in some improvements for the -Onone performance:
RangeAssignment: +31%
StdlibSort: +20%




Swift SVN r25123