[PowerPC] Fix td pattern for P10 VSLDBI and VSRDBI

This patch fixes the pattern for the P10 instructions Vector Shift Left
Double by Bit Immediate VN-form and Vector Shift Right Double by Bit
Immediate VN-form. The third argument should be a target constant (`timm`)
instead of an `i32` because an immediate is expected.

Reviewed By: lei

Differential Revision: https://reviews.llvm.org/D109920