[ARM] Fix undefined behaviour in bf16->float conversion (#116985)

This was implementing the bf16->float conversion function using a
left-shift of a signed integer, so for negative floating-point values a
1 was being shifted into the sign bit of the signed integer intermediate
value. This is undefined behaviour, and was caught by UBSan.

The vector versions are code-generated via Neon builtin functions, so
probably don't have the same UB problem, but I've updated them anyway to
be consistent.

Fixes #61983.