RFR(M): 8213528: fp registers should not need to be saved around a CallLeafNoFP

[Roland Westrelin]

> Save/restore XMM registers in a wrapper stub is perfectly fine with me. May be we should name such calls differently - 
> NO_FP is misleading.

Rather than a different node type, what about a field in CallLeafNode
(bool _preserve_fp_registers;)? I would then leave both CallLeafNode and
CallLeafNoFpNode alone for now and the patch would simply test for
_preserve_fp_registers in lcm.cpp?

Roland.