I don’t understand the ‘+’ in the comment "# r10 = r10 (0x64) + 0xFFFFFF80”, and it could be a typo. But I understand the comment on the next line, "# r31 = r11 (0xE3) & 0xFFFFFF80”.
If you look at the PowerPC 2.02 Book 1 documentation for 'Rotate Left Word Immediate then AND with Mask M-form’, it shows what is actually done for clrrwi.
clrrwi Rx,Ry,n
is equivalent to
rlwinm Rx,Ry,0,0,31-n
For clrrwi, rlwinm rotates left the word 0 bits, so there is no rotate done for clrrwi instructions. It generates a mask with 1’s in bits 0-24 and zero elsewhere (bits 25-31), so the mask generated for n=7 is 0xFFFFFF80. The rotated data are ANDed with the generated mask, so this is the reason that the comment for the second clrrwi makes sense. clrrwi is generating a mask to clear the right n bits in a word, and 0xFFFFFF80 is the mask used to do the clear.
By the way, 0x34+0x7F=0xB3.