Rework subreg_get_info

[Richard Sandiford]

Richard Sandiford <richard.sandiford@arm.com> writes:
> This isn't intended to change the behaviour, just rewrite the
> existing logic in a different (and hopefully clearer) way.
> The new form -- particularly the part based on the "block"
> concept -- is easier to convert to polynomial sizes.
>
> Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?

Sorry, I should have said: this was also tested by compiling the
testsuite before and after the change at -O2 -ftree-vectorize on:

    aarch64-linux-gnueabi alpha-linux-gnu arc-elf arm-linux-gnueabi
    arm-linux-gnueabihf avr-elf bfin-elf c6x-elf cr16-elf cris-elf
    epiphany-elf fr30-elf frv-linux-gnu ft32-elf h8300-elf
    hppa64-hp-hpux11.23 ia64-linux-gnu i686-pc-linux-gnu
    i686-apple-darwin iq2000-elf lm32-elf m32c-elf m32r-elf
    m68k-linux-gnu mcore-elf microblaze-elf mips-linux-gnu
    mipsisa64-linux-gnu mmix mn10300-elf moxie-rtems msp430-elf
    nds32le-elf nios2-linux-gnu nvptx-none pdp11 powerpc-linux-gnu
    powerpc-eabispe powerpc64-linux-gnu powerpc-ibm-aix7.0 rl78-elf
    rx-elf s390-linux-gnu s390x-linux-gnu sh-linux-gnu sparc-linux-gnu
    sparc64-linux-gnu sparc-wrs-vxworks spu-elf tilegx-elf tilepro-elf
    xstormy16-elf v850-elf vax-netbsdelf visium-elf x86_64-darwin
    x86_64-linux-gnu xtensa-elf

There were no differences in assembly output.

Thanks,
Richard