RFR (11): Cleaner separation between Shenandoah and other GCs in C1's compare-and-swap generation

[Roman Kennke]

Looking at this here:

https://builds.shipilev.net/patch-openjdk-shenandoah-jdk11/src/hotspot/cpu/x86/c1_LIRGenerator_x86.cpp.udiff.html

I see that we're unconditionally allocating 2 tmp registers for c1's
CAS, because we need them for Shenandoah. We better separate this to
compile only with INCLUDE_SHENANDOAHGC and when UseShenandoahGC, so that
other GCs don't get punished by Shenandoah-only requirement:

http://cr.openjdk.java.net/~rkennke/jdk11-better-c1-cas-regalloc/webrev.00/

Ok?

Roman