Inline refactor: revise checks in fgFindJumpTargets

Inline refactor: revise checks in fgFindJumpTargets

In this method, the inliner will check for unsupported opcodes and
similar things. Refactor these checks to directly bail out if an
issue is discovered, rather than going to a common handler snippet.

There are two functionality changes:

First, if an unsupported opcode is found (currently: switch, jmp,
calli, localloc, cpblk, mkrefany, rethrow) mark the inlinee as
never inlinable, rather than just failing the current attempt.
This should not change externally visible behavior but will improve
jit TP marginally if such cases are common.

Second, for NETCF quirk handling, make sure to guard some of the
observations with a check that we're reading the IL for inlining.

Commit migrated from https://github.com/dotnet/coreclr/commit/ebbea0a31a72c0d455dc3124a88e400fb4b2c510