[x86] Cache result of expensive_function_p between frame layouts

[x86] Cache result of expensive_function_p between frame layouts

ix86_compute_frame_layout sets use_fast_prologue_epilogue if
the function isn't more expensive than a certain threshold,
where the threshold depends on the number of saved registers.
However, the RA is allowed to insert and delete instructions
as it goes along, which can change whether this threshold is
crossed or not.

I hit this with an RA change I'm working on.  Rematerialisation
was able to remove an instruction and avoid a spill, which happened
to bring the size of the function below the threshold.  But since
nothing legitimately frame-related had changed, there was no need for
the RA to lay out the frame again.  We then failed the final sanity
check in lra_eliminate.

2019-09-30  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/i386/i386.h (ix86_frame::expensive_p): New field.
	(ix86_frame::expensive_count): Likewise.
	* config/i386/i386.c (ix86_compute_frame_layout): Make the choice
	of use_fast_prologue_epilogue robust against incidental changes
	in function size.

From-SVN: r276361