[SDPatternMatch] Only match ISD::SIGN_EXTEND in m_SExt

[c8ef]

Context: #95365 (comment)

The current implementation of m_SExt matches both ISD::SIGN_EXTEND and ISD::SIGN_EXTEND_INREG. However, in cases where we specifically need to match only ISD::SIGN_EXTEND, such as in the SelectionDAG graph below, this can lead to issues and unintended combinations.

SelectionDAG has 13 nodes:
  t0: ch,glue = EntryToken
          t2: v2i32,ch = CopyFromReg t0, Register:v2i32 %0
        t21: v2i32 = sign_extend_inreg t2, ValueType:ch:v2i8
          t4: v2i32,ch = CopyFromReg t0, Register:v2i32 %1
        t22: v2i32 = sign_extend_inreg t4, ValueType:ch:v2i8
      t23: v2i32 = avgfloors t21, t22
    t24: v2i32 = sign_extend_inreg t23, ValueType:ch:v2i8
  t15: ch,glue = CopyToReg t0, Register:v2i32 $d0, t24
  t16: ch = AArch64ISD::RET_GLUE t15, Register:v2i32 $d0, t15:1

[RKSimon]

Cleanup the PR title and summary. Title should be something like "[SDPatternMatch] Only match ISD::SIGN_EXTEND in m_SExt" and then refer to #90762 in the summary and explain why we need this.

[c8ef]

Cleanup the PR title and summary. Title should be something like "[SDPatternMatch] Only match ISD::SIGN_EXTEND in m_SExt" and then refer to #90762 in the summary and explain why we need this.

Fixed. I will be more careful with the PR description and title in the future.

[topperc]

LGTM