initblk incorrectly handles 0 fill on ARM64

[mikedn]

The fill values passed to initblk is supposed to be a byte but ARM64 block store lowering fails to mask the upper bytes resulting in incorrect initialization:

ldloca V_0
ldc.i4 0x11100
ldc.i4 15
initblk

generates:

52822000          movz    w0, #0x1100
72A00020          movk    w0, dotnet/coreclr#1 LSL dotnet/coreclr#16
9101C3A1          add     x1, fp, dotnet/coreclr#112  // [V05 loc0]
F9000020          str     x0, [x1]     ; stores 0x0000000000011100
B9000820          str     w0, [x1,#8]  ; stores 0x00011100
79001820          strh    w0, [x1,#12] ; stores 0x1100
39003820          strb    w0, [x1,#14] ; stores 0x00

dotnet/coreclr#27035 (comment)