[X86] Tighten up a comment which confused x64 ABI terminology.

The x64 ABI has two major function types:
 - frame functions
 - leaf functions

A frame function is one which requires a stack frame.  A leaf function
is one which does not.  A frame function may or may not have a frame
pointer.

A leaf function does not require a stack frame and may never modify SP
except via a return (RET, tail call via JMP).

A frame function which has a frame pointer is permitted to use the LEA
instruction in the epilogue, a frame function without which doesn't
establish a frame pointer must use ADD to adjust the stack pointer epilogue.

Fun fact: Leaf functions don't require a function table entry
(associated PDATA/XDATA).

llvm-svn: 281006