Add optimized ICs for new CanvasArray types introduced in WebGL

Add optimized ICs for new CanvasArray types introduced in WebGL
specification under development. This is a follow-on CL to
http://codereview.chromium.org/293023 .

Based on review feedback, defined the behavior of storing NaN and
+/-Infinity into external arrays of integer types as storing 0. Added
test cases. Added fucomi instruction to assembler. Fixed bug in
KeyedLoadIC::GenerateExternalArray when allocation of HeapNumber
failed. Fixed bug in encoding of 16-bit immediate arithmetic
instructions in 64-bit port.

Removed raising of exceptions for negative array indices passed to
external arrays and associated tests. Based on current discussion in
WebGL working group, will probably end up removing the exception
throwing altogether.

Review URL: http://codereview.chromium.org/294022

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3113 ce2b1a6d-e550-0410-aec6-3dcde31c8c00