[IR] Switch to more normal template parameter names ending in `T` instead of `Ty`.
[IR] Switch to more normal template parameter names ending in `T`
instead of `Ty`.

The `Ty` suffix is much more commonly used for LLVM `Type` variable
names, so this seemed like a particularly confusing collision.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@298808 91177308-0d34-0410-b5e6-96231b3b80d8

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