analyze: misc fixes for lighttpd buffer (part 1/2)

[spernsteiner]

Various small fixes for lighttpd's buffer module:

• In rewrite::expr::mir_op, skip generating casts for Deref projections on FIXED pointers
• In borrowck, don't treat StorageLive/StorageDead as uses (which would invalidate loans)
• In rewrite::ty::HirTyVisitor, add a missing walk_stmt call. This was causing type rewrites to be skipped in some places.
• In rewrite::expr::mir_op, add a conversion from &[T] to &T before dereferencing if needed. This can happen in cases like p.offset(...); x = *p; because *p uses p directly, which has the OFFSET permission due to the previous p.offset() call, instead of copying p into a temporary, which would not need the OFFSET permission.
• In GlobalAnalysisCtxt::fn_analysis_invalid, ignore the flag DontRewriteFnReason::REWRITE_INVALID, since rewriting errors don't affect the validity of the analysis results.