In the beginning, Ice::Inst was called IceInst, and patterns like "IceInst *Inst = ..." made perfect sense.

After the Ice:: name change, "Inst *Inst = ..." continued to compile, mostly.

However, shadowing a type name is clumsy and newer code tends to use "Inst *Instr", so we might as well switch all the instances over.

Some are still called "I" and those are left alone.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/1665423002 .