tree: avoid creating new DString in AdjustValueToType when possible

Previously, in `AdjustValueToType` we would always create a new
`DString` object if the original datum wasn't modified in any way. This
seems unnecessary, so we now track whether a modification might have
occurred and allocate the new object only in that case, otherwise
returning the original datum. We choose to not do the same for collated
strings since there it wouldn't have been an equivalent change (we
currently initialize a new CollationEnvironment).

Noticed this inefficiency when looking at CPU profile of a node running
an IMPORT.
```
name                                old time/op    new time/op    delta
ImportFixture/tpcc/warehouses=1-24     586ms ± 9%     585ms ± 4%     ~     (p=0.853 n=10+10)

name                                old speed      new speed      delta
ImportFixture/tpcc/warehouses=1-24   148MB/s ±10%   148MB/s ± 5%     ~     (p=0.912 n=10+10)

name                                old alloc/op   new alloc/op   delta
ImportFixture/tpcc/warehouses=1-24    1.13GB ± 1%    1.10GB ± 1%   -2.76%  (p=0.000 n=10+10)

name                                old allocs/op  new allocs/op  delta
ImportFixture/tpcc/warehouses=1-24     6.08M ± 0%     4.12M ± 0%  -32.25%  (p=0.000 n=10+10)
```

Release note: None

Author: yuzefovich

pkg/sql/sem/tree/datum.go

@@ -6393,11 +6393,15 @@ func AdjustValueToType(typ *types.T, inVal Datum) (outVal Datum, err error) {
 	switch typ.Family() {
 	case types.StringFamily, types.CollatedStringFamily:
 		var sv string
+		var isString, isCollatedString bool
 		if v, ok := AsDString(inVal); ok {
 			sv = string(v)
+			isString = true
 		} else if v, ok := inVal.(*DCollatedString); ok {
 			sv = v.Contents
+			isCollatedString = true
 		}
+		origLen := len(sv)
 		switch typ.Oid() {
 		case oid.T_char:
 			// "char" is supposed to truncate long values.
@@ -6432,9 +6436,14 @@ func AdjustValueToType(typ *types.T, inVal Datum) (outVal Datum, err error) {
 		}
 
 		if typ.Oid() == oid.T_bpchar || typ.Oid() == oid.T_char || typ.Oid() == oid.T_varchar {
-			if _, ok := AsDString(inVal); ok {
+			if isString {
+				if len(sv) == origLen {
+					// The string wasn't modified, so we can just return the
+					// original datum.
+					return inVal, nil
+				}
 				return NewDString(sv), nil
-			} else if _, ok := inVal.(*DCollatedString); ok {
+			} else if isCollatedString {
 				return NewDCollatedString(sv, typ.Locale(), &CollationEnvironment{})
 			}
 		}