[FastISel][AArch64] Fix "Fold sign-/zero-extends into the load instruction."

This commit fixes an issue with sign-/zero-extending loads that was discovered
by Richard Barton.

We use now the correct load instructions for sign-extending loads to 64bit. Also
updated and added more unit tests.
[FastISel][AArch64] Teach the address computation to also fold sub instructions.

Tiny enhancement to the address computation code to also fold sub instructions
if the rhs is constant and can be folded into the offset.
[FastISel][AArch64] Teach the address computation code to also fold sign-/zero-extends.

The code already folds sign-/zero-extends, but only if they are arguments to
mul and shift instructions. This extends the code to also fold them when they
are direct inputs.