[GlobalISel] Add a combine for sext_inreg(load x), c --> sextload x

This is restricted to single use loads, which if we fold to sextloads we can
find more optimal addressing modes on AArch64.

This also fixes an overload the MachineFunction::getMachineMemOperand() method
which was incorrectly using the MF alignment instead of the MMO alignment.

Differential Revision: https://reviews.llvm.org/D85966