[InstCombine] relax masking requirement for truncated funnel/rotate match

I was investigating a seemingly unrelated improvement in demanded
bits for shift-left, but that caused regressions on these tests
because we were able to look through/eliminate the mask.

https://alive2.llvm.org/ce/z/Ztdr22

  define i8 @src(i32 %x, i32 %y, i32 %shift) {
  %and = and i32 %shift, 3
  %conv = and i32 %x, 255
  %shr = lshr i32 %conv, %and
  %sub = sub i32 8, %and
  %shl = shl i32 %y, %sub
  %or = or i32 %shr, %shl
  %conv2 = trunc i32 %or to i8
  ret i8 %conv2
  }

  define i8 @tgt(i32 %x, i32 %y, i32 %shift) {
  %x8 = trunc i32 %x to i8
  %y8 = trunc i32 %y to i8
  %shift8 = trunc i32 %shift to i8
  %and = and i8 %shift8, 3
  %conv2 = call i8 @llvm.fshr.i8(i8 %y8, i8 %x8, i8 %and)
  ret i8 %conv2
  }

  declare i8 @llvm.fshr.i8(i8,i8,i8)