Fix doc for t inline asm constraints for ARM/Thumb

[pbarrio]

Summary

The constraint goes up to regs d15 and q7, not d16 and q8.

[pbarrio]

Discussing with @chill on a chat, he was happier with the following wording:

A 32, 64, or 128-bit floating-point/SIMD register in the ranges s0-s31, d0-d31, or q0-q15, respectively.

And so forth. I will change these in all SIMD constraints (w, t, x) in both Thumb1 and ARM/Thumb2 modes.

[pbarrio]

Reworded all the FP & SIMD constraints

llvm/docs/LangRef.rst

 - ``M``: A power of two or a integer between 0 and 32.
 - ``N``: Invalid immediate constraint.
 - ``O``: Invalid immediate constraint.
 - ``r``: A general-purpose 32-bit integer register (``r0-r15``).
 - ``l``: In Thumb2 mode, low 32-bit GPR registers (``r0-r7``). In ARM mode, same
   as ``r``.
 - ``h``: In Thumb2 mode, a high 32-bit GPR register (``r8-r15``). In ARM mode,
   invalid.
-- ``w``: A 32, 64, or 128-bit floating-point/SIMD register: ``s0-s31``,
-  ``d0-d31``, or ``q0-q15``.
-- ``x``: A 32, 64, or 128-bit floating-point/SIMD register: ``s0-s15``,
-  ``d0-d7``, or ``q0-q3``.
-- ``t``: A low floating-point/SIMD register: ``s0-s31``, ``d0-d16``, or
-  ``q0-q8``.
+- ``w``: A 32, 64, or 128-bit floating-point/SIMD register in the ranges
+  ``s0-s31``, ``d0-d31``, or ``q0-q15``, respectively.
+- ``t``: A 32, 64, or 128-bit floating-point/SIMD register in the ranges
+  ``s0-s31``, ``d0-d15``, or ``q0-q7``, respectively.
+- ``x``: A 32, 64, or 128-bit floating-point/SIMD register in the ranges
+  ``s0-s15``, ``d0-d7``, or ``q0-q3``, respectively.
 
 ARM's Thumb1 mode:
 
 - ``I``: An immediate integer between 0 and 255.
 - ``J``: An immediate integer between -255 and -1.
 - ``K``: An immediate integer between 0 and 255, with optional left-shift by
   some amount.
 - ``L``: An immediate integer between -7 and 7.
 - ``M``: An immediate integer which is a multiple of 4 between 0 and 1020.
 - ``N``: An immediate integer between 0 and 31.
 - ``O``: An immediate integer which is a multiple of 4 between -508 and 508.
 - ``r``: A low 32-bit GPR register (``r0-r7``).
 - ``l``: A low 32-bit GPR register (``r0-r7``).
 - ``h``: A high GPR register (``r0-r7``).
-- ``w``: A 32, 64, or 128-bit floating-point/SIMD register: ``s0-s31``,
-  ``d0-d31``, or ``q0-q15``.
-- ``x``: A 32, 64, or 128-bit floating-point/SIMD register: ``s0-s15``,
-  ``d0-d7``, or ``q0-q3``.
-- ``t``: A low floating-point/SIMD register: ``s0-s31``, ``d0-d16``, or
-  ``q0-q8``.
+- ``w``: A 32, 64, or 128-bit floating-point/SIMD register in the ranges
+  ``s0-s31``, ``d0-d31``, or ``q0-q15``, respectively.
+- ``t``: A 32, 64, or 128-bit floating-point/SIMD register in the ranges
+  ``s0-s31``, ``d0-d15``, or ``q0-q7``, respectively.
+- ``x``: A 32, 64, or 128-bit floating-point/SIMD register in the ranges
+  ``s0-s15``, ``d0-d7``, or ``q0-q3``, respectively.
 
 
 Hexagon:
 
 - ``o``, ``v``: A memory address operand, treated the same as constraint ``m``,
   at the moment.
 - ``r``: A 32 or 64-bit register.