[CodeGen] Make MachineInstr::isIdenticalTo() symmetric.

Summary:
MachineInstr::isIdenticalTo() is for some reason not
symmetric when comparing bundles, which gives us the
property:

  I1->isIdenticalTo(*I2) != I2->isIdenticalTo(*I1)

when comparing bundles where one bundle is longer than
the other.

This patch makes sure that bundles of different length
always are considered as not being identical. Thus, the
result of the comparison will be the same regardless of
which side that happens to be to the left.

Reviewers: dexonsmith, jonpa, andrew.w.kaylor

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D27508

llvm-svn: 290095