[Hexagon] Generate trap/undef if misaligned access is detected

This applies to memory accesses to (compile-time) constant addresses
(such as memory-mapped registers). Currently when a misaligned access
to such an address is detected, a fatal error is reported. This change
will emit a remark, and the compilation will continue with a trap,
and "undef" (for loads) emitted.

This fixes https://llvm.org/PR50838.

Differential Revision: https://reviews.llvm.org/D50524