Make the flag properties use '< 0'.

When interpreting the truth value of an integer in a SIMD value, use the
'< 0' convention.
Clarify comments and code.

More firmly establish the convention of converting from boolean to
integer.
Elementwise select.

This renames the existing bitwise select to bitselect, and introduces a
new elementwise select, following the convention for boolean values in
SIMD of examining the sign bit.
Simplify signMask implementations.
Rename shuffle to swizzle

This is in preparation for replacing shuffleMix with a fully-general
two-input shuffle, to be named 'shuffle'.
Replace shuffleMix with a fully-general shuffle

https://github.com/johnmccutchan/ecmascript_simd/issues/74
Tests for int32x4 shuffle and swizzle
Switch from swizzle masks to swizzle indices

This changes SIMD.float32x4.swizzle(x, SIMD.XYZW) to
SIMD.float32x4.swizzle(0, 1, 2, 3). This gives us scalability for
int8x16, and consistency with SIMD.float32x4.shuffle, which has already
made this transition.
Merge pull request #83 from sunfishcode/shuffle

New-style shuffles
Merge pull request #86 from sunfishcode/int32-to-bool

Boolean conventions
Fix a missing '}'
Initial int16x8 and int8x16 support

https://github.com/johnmccutchan/ecmascript_simd/issues/75
Merge pull request #92 from sunfishcode/master

Int16x8 and Int8x16 support
Added missing int32x4 comparison operations

Fixes #46
Comments: Add missing params in functions comments.
Comments: Use {integer} rather than {int} to describe the integer type.
Comments: typo.
Added tests for int32x4.{notEqual,lessThanOrEqual,greaterThanOrEqual}
Merge pull request #49 from ProgramFOX/master

Added missing int32x4 comparison operations, fixes #46
Merge pull request #100 from bnjbvr/cleanup-comments

Cleanup comments