vcompress and mask-producing instr tail elements signature fix

[georgiatai]

Fixes on signature for vcompress and mask-producing instruction tail elements as mentioned in issue #1148

[georgiatai]

Add indentations so signature.h is a little unclear on what is changed, the main changes are line 368-369, 375-379, 409-416, and the corresponding non-selfchecking version.

This PR was closed and reopened because there are things that I thought I haven't fix but I actually did. Sorry for the confusion.

[Jimx-]

It looks like vcompress is not the only instruction that can have a different effective vector length of vd than the original vl. vmv.s.x and all reduction operations write scalar result to element 0 of vd and other elements (0 < index < VLEN/VSEW) are tail elements. They are like vcompress but with only 1 active element in vd.