The current X86 NOP padding uses one long NOP followed by the remainder in
one-byte NOPs.  If the processor actually executes those NOPs, as it sometimes
does with aligned bundling, this can have a performance impact.  From my
micro-benchmarks run on my one machine, a 15-byte NOP followed by twelve
one-byte NOPs is about 20% worse than a 15 followed by a 12.  This patch
changes NOP emission to emit as many 15-byte (the maximum) as possible followed
by at most one shorter NOP.