[GlobalISel][AArch64] Add support for base register + offset register loads

Add support for folding G_GEPs into loads of the form

```
ldr reg, [base, off]
```

when possible. This can save an add before the load. Currently, this is only
supported for loads of 64 bits into 64 bit registers.

Add a new addressing mode function, `selectAddrModeRegisterOffset` which
performs this folding when it is profitable.

Also add a test for addressing modes for G_LOAD.

Differential Revision: https://reviews.llvm.org/D64944

llvm-svn: 366503