Clean up and improve documentation of x86_32 registers (#1629)

* Improve comments for register lengths

* Separate MMX/3DNow! and x87 register lengths and categories

The MMX* registers are only the lower 64 bits of the st* ones.

* Improve comments for EFLAGS fields

* Add conversion to ROSE register for FLAGC

FLAGC is the lower bit of the I/O Permission Level field in EFIELD.

* Add conversion to ROSE register for FLAGD

FLAGC is the upper bit of the I/O Permission Level field in EFIELD.

* Add conversion to ROSE register for Nested Task flag

* Add conversion to ROSE register for Resume Flag

* Add missing Virtual-8086 mode (VM) EFLAGS field

* Add missing Alignment Check/Access Control (AC) EFLAGS field

* Add missing Virtual Interrupt Flag (VIF) EFLAGS field

* Add missing Virtual Interrupt Pending (VIP) EFLAGS field

* Add missing ID Flag (ID) EFLAGS field

* Remove registers xmm8-xmm31 and aliases

These registers are only available in 64-bit mode.

From Intel(r) 64 and IA-32 Architectures Software Developer’s Manual
June 2021:

11.2.1 SSE2 in 64-Bit Mode and Compatibility Mode
  In compatibility mode, SSE2 extensions function like they do in
  protected mode. In 64-bit mode, eight additional XMM registers are
  accessible. Registers XMM8-XMM15 are accessed by using REX prefixes.

14.1.1 256-Bit Wide SIMD Register Support
  Intel AVX introduces support for 256-bit wide SIMD registers
  (YMM0-YMM7 in operating modes that are 32-bit or less, YMM0-YMM15 in
  64-bit mode).

15.1.2 32 SIMD Register Support
  Intel AVX-512 instructions also support 32 SIMD registers in 64-bit
  mode (XMM0-XMM31, YMM0-YMM31 and ZMM0-ZMM31). The number of available
  vector registers in 32-bit mode is still 8.

* Fix avx-512 opmask size.

It's 64 bits, not 128.

From Intel(R) 64 and IA-32 Architectures Software Developer’s Manual
June 2021

15.6.1 OPMASK Register to Predicate Vector Data Processing
  The opmask is a set of eight architectural registers of size
  MAX_KL (64-bit).

* Rename OCT to XMMS

This makes it consistent with the names used for the other vector
extensions.

* Use symbolic names for the segment register base IDs

* Add missing ROSE category conversions

* Add missing ROSE subrange conversions

* Preserve register number in getBaseRegister

* Clear whole subrange byte for GPRs in getBaseRegister