[AArch64][GlobalISel] Handle any-extending FPR loads in manual selection code.

[AArch64][GlobalISel] Handle any-extending FPR loads in manual selection code.

When we have an any-extending FPR bank load, none of the tablegen patterns
match and we fall back to the C++ selector. Like with the truncating stores
that were fixed recently, the C++ wasn't able to handle it and ended up
generating invalid copies between different size regclasses.

This change adds handling for this case, splitting the load into a regular
load and a SUBREG_TO_REG to extend it into the original wide destination reg.