[AArch64][GlobalISel] Don't match thread-local globals in matchFoldGlobalOffset

SelectionDAG has separate ISD opcodes for regular global values and thread-local
global values, while GlobalISel does not.

This combine was ported from SDAG directly without knowing that. As a result,
it was running on TLS globals.

This makes it so that `matchFoldGlobalOffset` doesn't match on TLS globals, and
adds an assert to `selectTLSGlobalValue` to make sure that TLS globals never
have offsets.

Differential Revision: https://reviews.llvm.org/D101478