[AArch64] Tie source and destination operands for AESMC/AESIMC.

Summary:
Most CPUs implementing AES fusion require instruction pairs of the form
    AESE Vn, _
    AESMC Vn, Vn
and
    AESD Vn, _
    AESIMC Vn, Vn

The constraint is added to AES(I)MC instructions which use the result of
an AES(E|D) instruction by using AES(I)MCTrr pseudo instructions, which
constraint source and destination registers to be the same.

A nice side effect of this change is that now all possible pairs are
scheduled back-to-back on the exynos-m1 for the misched-fusion-aes.ll
test case.

I had to update aes_load_store. The version I added initially was very
reduced and with the new constraint, AESE/AESMC could not be scheduled
back-to-back. I updated the test to be more realistic and still expose
the same scheduling problem as the initial test case.

Reviewers: t.p.northover, rengolin, evandro, kristof.beyls, silviu.baranga

Reviewed By: t.p.northover, evandro

Subscribers: aemerson, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D35299

llvm-svn: 309495