Clarify which type "Label" refers to (generic vs X86)

Clarify which type "Label" refers to (generic vs X86)

There is a generic Label that can be used as-is for ARM
and MIPS, and there is an x86 derived class Label which
adds the concept of near and far (for 8-bit vs 32-bit
jumps). Previously, one method "getOrCreateCfgNodeLabel"
would say that it returns a Label when it means the generic
one in some cases and the x86 one in other cases.

Split that into getCfgNodeLabel and getOrCreateCfgNodeLabel
where getCfgNodeLabel returns the generic one and
getOrCreateCfgNodeLabel is part of the x86 code and
returns the x86 one.

BUG=none
R=ascull@google.com, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1265023003 .