[RFC PATCH, i386]: Improve STV pass by correcting the cost of moves to/from XMM reg

[Uros Bizjak]

This is currently a heads-up patch that removes the minimum limitation
of cost of moves to/from XMM reg. The immediate benefit is the removal
of mismatched spills, caused by subreg usage.

*If* the patch proves to be beneficial (as in "doesn't regress
important benchmarks"), then we should be able to un-hide the
inter-regset moves from RA and allow it to collapse some moves. As an
example, patched compiler removes a movd in gcc.target/i386/minmax-6.c
and still avoids mismatched spill.

2019-08-23  Uroš Bizjak  <ubizjak@gmail.com>

    * config/i386/i386.c (ix86_register_move_cost): Do not
    limit the cost of moves to/from XMM register to minimum 8.
    * config/i386/i386-features.c
    (general_scalar_chain::make_vector_copies): Do not generate
    zeroing move from GPR to XMM register, use gen_move_insn
    instead of gen_gpr_to_xmm_move_src.
    (general_scalar_chain::convert_op): Ditto.
    (gen_gpr_to_xmm_move_src): Remove.

The patch was bootstrapped and regression tested on x86_64-linux-gnu
{,-m32}, configured w/ and w/o -with-arch=ivybridge.

The patch regresses PR80481 scan-asm-not (where the compiler generates
unrelated XMM spill on register starved x86_32). However, during the
analysis, I found that the original issue is not fixed, and is still
visible without -funrol-loops [1].

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80481#c10

So, I'd wait for the HJ's benchmark results of the cost to/from XMM
change before proceeding with the patch.

Uros.