[15] RFR (M): 8237581: Improve allocation expansion

[Nils Eliasson]

Hi,

This patch improves expand_allocate_common when there is no use of the 
allocation.

Three cases are improved:

- With unknown allocation length - don't expand the fast path. No 
allocation is needed when it isn't used. NegativeArraySizeException will 
still be caught by the slowpath.
- With known length inside the legal range - No fast path or slow path 
is needed. The allocation node is removed.
- With known length outside the legal range - only a slow path is needed.

I also refactored the code a bit, keeping the general structure for easy 
diff, but extracting some routines too make it more readable.

Bug: https://bugs.openjdk.java.net/browse/JDK-8237581
Webrev: http://cr.openjdk.java.net/~neliasso/8237581/webrev.03/

Please review!

Best regards,
Nils Eliasson