[BOLT][AArch64] Replace NOP with adrp in AdrRelaxationPass to preserve relative offsets.

Avoid replacing one adr instruction with two adrp+add by utilizing linker-provided nops
when they are present. By doing so we preserve relative offsets of next instructions
in a function which reduces chances to break undetected jump tables. This commit makes
release-mode lld-linked clang, lld and etc work after BOLT.

Reviewed By: rafauler, yota9

Differential Revision: https://reviews.llvm.org/D143887