[JIT] Recognize when length is never negative for Span<T>::.ctor(void*, int32) #83248
Description
I noticed this for the JIT intrinsic BitOperations.PopCount, for which the return value should be treated as never negative (since #64951).
unsafe Span<byte> M0(byte* p, byte b) => new(p, b);
unsafe Span<byte> M2(byte* p, int b) => new(p, (byte)b);
unsafe Span<byte> M3(byte* p, uint mask) => new(p, BitOperations.PopCount(mask));
unsafe Span<byte> M4(byte* p, uint mask) => new(p, (byte)BitOperations.PopCount(mask));// crossgen2 8.0.0-preview.3.23159.99+1b2eb12a711410a358bdfa93af017a34a929d4cf
C:M0(ulong,ubyte):System.Span`1[ubyte]:this:
; Emitting BLENDED_CODE for X64 CPU with AVX - Unix
push rax
;; size=1 bbWeight=1 PerfScore 1.00
mov rax, rsi
movzx rdx, dl
;; size=6 bbWeight=1 PerfScore 0.50
add rsp, 8
ret
;; size=5 bbWeight=1 PerfScore 1.25
C:M02(ulong,int):System.Span`1[ubyte]:this:
; Emitting BLENDED_CODE for X64 CPU with AVX - Unix
push rax
;; size=1 bbWeight=1 PerfScore 1.00
movzx rdx, dl
test edx, edx
jl SHORT G_M62596_IG04
mov rax, rsi
;; size=10 bbWeight=1 PerfScore 1.75
add rsp, 8
ret
;; size=5 bbWeight=1 PerfScore 1.25
G_M62596_IG04:
call [System.ThrowHelper:ThrowArgumentOutOfRangeException()]
int3
;; size=7 bbWeight=0 PerfScore 0.00
C:M1(ulong,uint):System.Span`1[ubyte]:this:
; Emitting BLENDED_CODE for X64 CPU with AVX - Unix
push rax
;; size=1 bbWeight=1 PerfScore 1.00
popcnt edx, edx
jl SHORT G_M53218_IG04
mov rax, rsi
;; size=9 bbWeight=1 PerfScore 3.25
add rsp, 8
ret
;; size=5 bbWeight=1 PerfScore 1.25
G_M53218_IG04:
call [System.ThrowHelper:ThrowArgumentOutOfRangeException()]
int3
;; size=7 bbWeight=0 PerfScore 0.00
C:M2(ulong,uint):System.Span`1[ubyte]:this:
; Emitting BLENDED_CODE for X64 CPU with AVX - Unix
push rax
;; size=1 bbWeight=1 PerfScore 1.00
popcnt edx, edx
jl SHORT G_M40097_IG04
mov rax, rsi
;; size=9 bbWeight=1 PerfScore 3.25
add rsp, 8
ret
;; size=5 bbWeight=1 PerfScore 1.25
G_M40097_IG04:
call [System.ThrowHelper:ThrowArgumentOutOfRangeException()]
int3
;; size=7 bbWeight=0 PerfScore 0.00
``