Performance regression

[Jim Wilson]

This has been discussed many times before.  The only way to avoid performance
loss from changing the semantics of LOAD_EXTEND_OP is to add new optimizer
support for sign/zero extend operations.

Since we went ahead with the LOAD_EXTEND_OP change to fix optimizer bugs,
it would be useful if someone was looking into the new optimization support
we need.  Maybe it can go on the projects list?  Add support for optimizing
sign/zero extend operations.  Some problems can be fixed by being careful
in the md file, but some problems can be solved only with an optimization
pass that is designed to optimize away redundant sign/zero extend operations.

Jim