This patch implements a target independent DAG combine to produce multiply-high

instructions from shifts. This DAG combine will combine shifts for any type as

long as the MULH on the narrow type is legal.

For now, it is enabled on PowerPC as PowerPC is the only target that has an

implementation of the `isMulhCheaperThanMulShift` TLI hook introduced in

D78271.

Moreover, this DAG combine focuses on catching the pattern:

(shift (mul (ext <narrow_type>:$a to <wide_type>), (ext <narrow_type>:$b to <wide_type>)), <narrow_width>)

to produce `mulhs` when we have a sign-extend, and `mulhu` when we have

a zero-extend.

**The patch performs the following checks:**

- Operation is a right shift arithmetic (
`sra`) or logical (`srl`) - Input to the shift is a multiply
- Both operands to the shift are sext/zext nodes
- The extends into the multiply are both the same
- The narrow type is half the width of the wide type
- The shift amount is the width of the narrow type
- The respective mulh operation is legal