[unroll] Avoid the "Insn" abbreviation of Instruction. This is quite

hard to type and read for me, and is inconsistent with the other
abbreviation in the base class "Inst". For most of these (where they are
used widely) I prefer just spelling it out as Instruction. I've changed
two of the short-lived variables to use "Inst" to match the base class.

llvm-svn: 229028