Restructure compiler/intrinsics/TestRotate.java for easier compilation

It seems that after JDK-8256438 this test is very slow. See JDK-8262465 for more evidence of this.
I believe the underlying cause is that main() is way too busy with test cases, so this eventually happens:

$ javac test/hotspot/jtreg/compiler/intrinsics/TestRotate.java
$ java -XX:-TieredCompilation -XX:CompileThreshold=1000 -Xbatch -XX:+PrintCompilation -cp test/hotspot/jtreg compiler.intrinsics.TestRotate
 ...
   6112 112 % !b compiler.intrinsics.TestRotate::main @ 1494 (3008 bytes) COMPILE SKIPPED: out of nodes during split (retry at different tier)

So, while the test is intended to test rotate intrinsics, I think it never gets there because compilation bails. And this also makes the test slow. This is tier1 test, and so its performance is somewhat important for developer experience.

This improvement restructures the test to be easier to JIT compile.