Request for reviews (S): 7048332: Cadd_cmpLTMask doesn't handle 64-bit tmp register properly

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/7048332/webrev

Fixed 7048332: Cadd_cmpLTMask doesn't handle 64-bit tmp register properly

enc_cmpLTP_mem() code relied on tmp be bound to rcx register but after 6511991 
changes it was not true any more. Use ins_encode %{ %} form to encode 
cadd_cmpLTMask() instruction and remove unused code.

Added regression test.