Rollup merge of #140034 - RalfJung:simd_select_bitmask-padding, r=workingjubilee

simd_select_bitmask: the 'padding' bits in the mask are just ignored

Fixes rust-lang/rust#137942: we documented simd_select_bitmask to require the 'padding' bits in the mask (the mask can sometimes be longer than the vector; I am referring to these extra bits as 'padding' here)  to be zero, mostly because nobody felt like doing the research for what should be done when they are non-zero. However, codegen is already perfectly happy just ignoring them, so in practice they can have any value. Some of the intrinsic wrappers in stdarch have trouble ensuring that they are zero. So let's just adjust the docs and Miri to permit non-zero 'padding' bits.

Cc ````@Amanieu```` ````@workingjubilee````

Author: GuillaumeGomez