[AArch64][GlobalISel] Fix incorrect handling of fp truncating stores.

[AArch64][GlobalISel] Fix incorrect handling of fp truncating stores.

When the tablegen patterns fail to select a truncating scalar FPR store,
our manual selection code also failed to handle it silently, trying to
generate an invalid copy. Fix this by adding support in the manual code
to generate a proper subreg copy before selecting a non-truncating store.