"BFI $0, R1, $7, R2" is expected to copy bit 0~6 from R1 to R2, and
left R2's other bits unchanged.

But the assembler rejects it with error "illegal bit number", and
BFIW/SBFIZ/SBFIZW/UBFIZ/UBFIZW have the same problem.

This CL fixes that issue and adds corresponding test cases.

fixes #26736

Change-Id: Ie0090a0faa38a49dd9b096a0f435987849800b76
Reviewed-on: https://go-review.googlesource.com/127159
Run-TryBot: Ben Shi <powerman1st@163.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>